pgsql: Split the buffer mapping table into multiple separately lockable

Log Message:
-----------
Split the buffer mapping table into multiple separately lockable
partitions, as per discussion. Passes functionality checks, but
I don't have any performance data yet.

Modified Files:
--------------
pgsql/contrib/pg_buffercache:
pg_buffercache_pages.c (r1.7 -> r1.8)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/pg_buffercache/pg_buffercache_pages.c.diff?r1=1.7&r2=1.8)
pgsql/src/backend/storage/buffer:
README (r1.10 -> r1.11)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/buffer/README.diff?r1=1.10&r2=1.11)
buf_table.c (r1.46 -> r1.47)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/buffer/buf_table.c.diff?r1=1.46&r2=1.47)
bufmgr.c (r1.208 -> r1.209)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/buffer/bufmgr.c.diff?r1=1.208&r2=1.209)
pgsql/src/backend/storage/lmgr:
lwlock.c (r1.40 -> r1.41)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/lmgr/lwlock.c.diff?r1=1.40&r2=1.41)
pgsql/src/include/storage:
buf_internals.h (r1.86 -> r1.87)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/storage/buf_internals.h.diff?r1=1.86&r2=1.87)
lock.h (r1.94 -> r1.95)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/storage/lock.h.diff?r1=1.94&r2=1.95)
lwlock.h (r1.28 -> r1.29)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/storage/lwlock.h.diff?r1=1.28&r2=1.29)