pgsql: Make TransactionIdIsInProgress check transam.c's single-item XID

Log Message:
-----------
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.

Modified Files:
--------------
pgsql/src/backend/access/transam:
transam.c (r1.73 -> r1.74)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/transam.c?r1=1.73&r2=1.74)
pgsql/src/backend/storage/ipc:
procarray.c (r1.40 -> r1.41)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/ipc/procarray.c?r1=1.40&r2=1.41)
pgsql/src/include/access:
transam.h (r1.64 -> r1.65)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/access/transam.h?r1=1.64&r2=1.65)