Make TransactionIdIsInProgress check transam.c's single-item XID status cache

Make TransactionIdIsInProgress check transam.c's single-item XID status cache
before it goes groveling through the ProcArray.  In situations where the same
recently-committed transaction ID is checked repeatedly by tqual.c, this saves
a lot of shared-memory searches.  And it's cheap enough that it shouldn't
hurt noticeably when it doesn't help.
Concept and patch by Simon, some minor tweaking and comment-cleanup by Tom.