The previous commit, made in anger, made a test pass, but wasn't
really the right fix.

This commit fixes MVCCMappingStorage's loadBefore implementation by
fixing handling of the internal _ltid variable so that it's updated
during poll_invalidations.  This allowed the base class version of
loadBefore to be used and, I'm 97% sure has the right semantics.
Fixing this revealed a problem with the Connection changes.

Fixed Connection.py to poll for invalidations before computing
_txn_time by calling lastTransaction, so as to get a current value.
We still apply invalidations after computing _txn_time, so that
persistent classes can be loaded correctly when they are invalidated.
This was accomplished by weaving _flush_invalidations into
newTransaction.