pgsql: Modify AtEOXact_CatCache and AtEOXact_RelationCache to assume

Log Message:
-----------
Modify AtEOXact_CatCache and AtEOXact_RelationCache to assume that the
ResourceOwner mechanism already released all reference counts for the
cache entries; therefore, we do not need to scan the catcache or relcache
at transaction end, unless we want to do it as a debugging crosscheck.
Do the crosscheck only in Assert mode.  This is the same logic we had
previously installed in AtEOXact_Buffers to avoid overhead with large
numbers of shared buffers.  I thought it'd be a good idea to do it here
too, in view of Kari Lavikka's recent report showing a real-world case
where AtEOXact_CatCache is taking a significant fraction of runtime.

Modified Files:
--------------
    pgsql/src/backend/access/transam:
        xact.c (r1.211 -> r1.212)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xact.c.diff?r1=1.211&r2=1.212)
    pgsql/src/backend/utils/cache:
        catcache.c (r1.121 -> r1.122)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/cache/catcache.c.diff?r1=1.121&r2=1.122)
        relcache.c (r1.225 -> r1.226)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/cache/relcache.c.diff?r1=1.225&r2=1.226)
    pgsql/src/backend/utils/resowner:
        resowner.c (r1.12 -> r1.13)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/resowner/resowner.c.diff?r1=1.12&r2=1.13)