pgsql-server: Tweak code so that pg_subtrans is never consulted for XIDs

Log Message:
-----------
Tweak code so that pg_subtrans is never consulted for XIDs older than
RecentXmin (== MyProc->xmin). This ensures that it will be safe to
truncate pg_subtrans at RecentGlobalXmin, which should largely eliminate
any fear of bloat. Along the way, eliminate SubTransXidsHaveCommonAncestor,
which isn't really needed and could not give a trustworthy result anyway
under the lookback restriction.
In an unrelated but nearby change, #ifdef out GetUndoRecPtr, which has
been dead code since 2001 and seems unlikely to ever be resurrected.

Modified Files:
--------------
pgsql-server/src/backend/access/transam:
subtrans.c (r1.1 -> r1.2)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/access/transam/subtrans.c.diff?r1=1.1&r2=1.2)
transam.c (r1.57 -> r1.58)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/access/transam/transam.c.diff?r1=1.57&r2=1.58)
pgsql-server/src/backend/storage/ipc:
sinval.c (r1.68 -> r1.69)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/storage/ipc/sinval.c.diff?r1=1.68&r2=1.69)
pgsql-server/src/backend/storage/lmgr:
lmgr.c (r1.66 -> r1.67)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/storage/lmgr/lmgr.c.diff?r1=1.66&r2=1.67)
pgsql-server/src/backend/utils/time:
tqual.c (r1.74 -> r1.75)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/utils/time/tqual.c.diff?r1=1.74&r2=1.75)
pgsql-server/src/include/access:
subtrans.h (r1.1 -> r1.2)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/include/access/subtrans.h.diff?r1=1.1&r2=1.2)
xlog.h (r1.54 -> r1.55)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/include/access/xlog.h.diff?r1=1.54&r2=1.55)