pgsql: Use ResourceOwners in the snapshot manager, instead of attempting

Log Message:
-----------
Use ResourceOwners in the snapshot manager, instead of attempting to track them
by hand. As an added bonus, the new code is smaller and more understandable,
and the ugly loops are gone.

This had been discussed all along but never implemented. It became clear that
it really needed to be fixed after a bug report by Pavan Deolasee.

Modified Files:
--------------
pgsql/src/backend/utils/resowner:
README (r1.7 -> r1.8)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/resowner/README?r1=1.7&r2=1.8)
resowner.c (r1.29 -> r1.30)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/resowner/resowner.c?r1=1.29&r2=1.30)
pgsql/src/backend/utils/time:
snapmgr.c (r1.6 -> r1.7)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/time/snapmgr.c?r1=1.6&r2=1.7)
pgsql/src/include/utils:
resowner.h (r1.15 -> r1.16)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/resowner.h?r1=1.15&r2=1.16)