This can be used to monitor the queue size in bytes rather than number of files

This can now be safely use to avoid overloading the queue.

This avoids having to record (and later retrieve) the pointer in a per-TClass global container to remember how to delete
the object.  Currently we leverage this in TVirtualArray

Note: If a StreamerInfo is loaded from a file and is the same information
as an existing one, it is assigned the same "unique id" and we need to double
check before removing it from the global list.

The TClass generated by TStreamerInfo::GenerateInfoForPair is neither 'Interpreted' (cling
likely does not, yet, have any information about that pair) nor loaded (the user did not
request a dictionary for it) but need to have special treatment for those TClass.

It is not clear whether this should be a new state.  For now we introduce a new
data member (`fIsSyntheticPair`) and don't increase the size of TClass instance
by using a bit field.

* Avoid calling TTree::ChangeFile if file is TMemFile
* [docs] Update documentation in TTree::Fill and TTree::ChangeFile
* Avoid triggering TTree::ChangeFile mechanism in RDF Snapshot

Fixes ROOT-10896.

Previously we issue a Fatal message about impossible code path.

The silent flag to skip over error message about transient data member.

Also remove last traces of R__GenerateTClassForPair

This fix #6726

As reported by CMSSW tests (for example: cms-sw/cmsdist#6314 (comment)) where the data appear odd/corrupted, there is an issue in TStreamerInfo::GenerateInfoForPair (which is almost always used for std::pair in the tip of v6.22 and master).

The problem is when calculating the offset of the second data member, TStreamerInfo::GenerateInfoForPair uses (unwittingly, of course :) ), the value zero for the size of the enums.

static specifier not required here

By chance internal data member is the same as "subtype" argument

While forward declaration changed, it need to be updated as well

Except in the case where we know the type did exist at some point and thus is an enum (because
it was not then we would have information about in the form of TStreamerInfo).

The exception to the exception is the case of a custom Streamer that does not use TStreamerInfo
underneath.  In that case, we don't know about the class then we don't have access to the custom
Streamer and thus can't read the object.

In particular use TEnum::GetEnum 'do-not-autoparse-nor-autoload' feature.

Now that TClingDataMemberInfo descends into unnamed unions, their members
must be "hidden" from ROOT IO. This fixes warnings such as

Warning in <TStreamerInfo::Build>: TString::Rep_t: TString::LongStr_t has no streamer or dictionary, data member "fLong" will not be saved

The implementation was not thread safe **and** has been obsolete ever since fType
is a TClassRef and since c9460ae5 make the update
much faster (single atomic update) and thread safe.

Rename TBasket::Reset to TBasket::WriteReset
Add TBasket::ReadResetBuffer which will shrink the buffer is it is currently significantly larger
than it is needed to any of the next (up to) 10 baskets.

Don't provoke auto-parsing during name normalization for the std::pair (but let it happen for its parameters if need be).

In TGenCollectionProxy Don't provoke autoparsing for an std::pair.

In TGenCollectionProxy handle the fact that we may now not find the std::pair.

Addresses (at least part of) ROOT-10932

Without this std::map<int,std::vector<double>* > got corrupted into std::map<int,std::vector<double,*> >.

This fixes ROOT-10796

TMapFile::WhichMapFile is called via operator delete which is used during RegisterModule
and letting TMapFile::WhichMapFile trigger the initialization of the TROOT object means that
  a) RegisterModule will have nested called to RegisterModule
  b) The interpreter will be created in the middle of the execution of RegisterModule.

Both are potentially fatal (i.e. undefined behavior).

Fixes assert in roottest/root/meta/MakeProject/runcms310.C
```
/builddir/build/BUILD/gcc-8.3.1-20190223/obj-x86_64-redhat-linux/x86_64-redhat-linux/libstdc++-v3/include/bits/basic_string.h:1067: std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::reference std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::operator[](std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::size_type) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>; std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::reference = char&; std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::size_type = long unsigned int]: Assertion '__pos <= size()' failed.
    at /mnt/build/wsincrmaster/LABEL/ROOT-fedora29/SPEC/python3/root/io/io/src/TStreamerInfo.cxx:3531
    subClasses=0x7fffffff8db0, top=true)
    at /mnt/build/wsincrmaster/LABEL/ROOT-fedora29/SPEC/python3/root/io/io/src/TStreamerInfo.cxx:3772
    subClasses=0x7fffffff8db0, extrainfos=0x7fffffff9210)
    at /mnt/build/wsincrmaster/LABEL/ROOT-fedora29/SPEC/python3/root/io/io/src/TStreamerInfo.cxx:3976
    at /mnt/build/wsincrmaster/LABEL/ROOT-fedora29/SPEC/python3/root/io/io/src/TFile.cxx:2843
```

where

```
(gdb) p inside
$1 = std::vector of length 3, capacity 4 = {"std::vector", "std::pair<edm::BranchKey,edm::BranchDescription>", ""}
```

Rationale
---------

We create a number of small root files to serve as conditions data. The
algorithm tells the framework to save the data to be valid for a given interval of
time.

To de-duplicate files we use the md5 checksum: same checksum, same
content, so no new file needed. Technically we create the file as a
temporary file, calculate the checksum and use a directory structure
similar to git objects: if no file with the checksum as name exists,
rename, otherwise just use the existing name.

For this to work we need the md5 checksum independent of the name of the
TFile. And since we rename the file anyway it really doesn't matter what
TFile thinks its name is.

Implementation
--------------

This tries to be the minimal change to the current feature of
reproducible files (thanks again): We piggyback on the same flag but if
the flag also has a value instead of jut being present we use it as the
name to put in the file.