pgsql: Remove BufferLocks[] array in favor of a single pointer to the

Log Message:
-----------
Remove BufferLocks[] array in favor of a single pointer to the buffer
(if any) currently waited for by LockBufferForCleanup(), which is all
that we were using it for anymore. Saves some space and eliminates
proportional-to-NBuffers slowdown in UnlockBuffers().

Modified Files:
--------------
pgsql/src/backend/storage/buffer:
buf_init.c (r1.68 -> r1.69)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/buffer/buf_init.c.diff?r1=1.68&r2=1.69)
bufmgr.c (r1.178 -> r1.179)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/buffer/bufmgr.c.diff?r1=1.178&r2=1.179)
pgsql/src/include/storage:
buf_internals.h (r1.73 -> r1.74)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/storage/buf_internals.h.diff?r1=1.73&r2=1.74)