pgsql: Give the ResourceOwner mechanism full responsibility for

Log Message:
-----------
Give the ResourceOwner mechanism full responsibility for releasing buffer
pins at end of transaction, and reduce AtEOXact_Buffers to an Assert
cross-check that this was done correctly.  When not USE_ASSERT_CHECKING,
AtEOXact_Buffers is a complete no-op.  This gets rid of an O(NBuffers)
bottleneck during transaction commit/abort, which recent testing has shown
becomes significant above a few tens of thousands of shared buffers.

Modified Files:
--------------
    pgsql/src/backend/access/transam:
        xact.c (r1.191 -> r1.192)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xact.c.diff?r1=1.191&r2=1.192)
    pgsql/src/backend/storage/buffer:
        bufmgr.c (r1.179 -> r1.180)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/buffer/bufmgr.c.diff?r1=1.179&r2=1.180)
        localbuf.c (r1.59 -> r1.60)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/buffer/localbuf.c.diff?r1=1.59&r2=1.60)
    pgsql/src/backend/storage/lmgr:
        proc.c (r1.154 -> r1.155)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/lmgr/proc.c.diff?r1=1.154&r2=1.155)
    pgsql/src/backend/utils/resowner:
        resowner.c (r1.7 -> r1.8)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/resowner/resowner.c.diff?r1=1.7&r2=1.8)
    pgsql/src/include/storage:
        bufmgr.h (r1.87 -> r1.88)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/storage/bufmgr.h.diff?r1=1.87&r2=1.88)