Better support for class that used to have version==0.

In the case where the class used to have a version number set to 0
(i.e. don't stream any of its content) but the version number is
now set to greater than zero, we need to avoid reading a non-existant
checksum from the buffer.
Add TBuffer::ReadVersionNoCheckSum

This (new) routine can be used from custom streamers
when it is known that the class was always versioned
(and thus never stored a checksum within the buffer).
This allows to disambiguate the case where the class
used to have a version number equal to zero and did
not save a byte count and is now versioned.

When reading the version number being zero, without
the byte count we have no way to guess whether the
class was version un-versioned or had previously a
version number equal to zero.