pgsql: Remove BufferBlockPointers array in favor of a base + (bufnum) *

Log Message:
-----------
Remove BufferBlockPointers array in favor of a base + (bufnum) * BLCKSZ
computation. On modern machines this is as fast if not faster, and we
don't have to clog the CPU's L2 cache with a tens-of-KB pointer array.
If we ever decide to adopt a more dynamic allocation method for shared
buffers, we'll probably have to revert this patch, but in the meantime
we might as well save a few bytes and nanoseconds. Per Qingqing Zhou.

Modified Files:
--------------
pgsql/src/backend/storage/buffer:
buf_init.c (r1.74 -> r1.75)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/buffer/buf_init.c.diff?r1=1.74&r2=1.75)
bufmgr.c (r1.192 -> r1.193)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/buffer/bufmgr.c.diff?r1=1.192&r2=1.193)
pgsql/src/include/storage:
bufmgr.h (r1.94 -> r1.95)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/storage/bufmgr.h.diff?r1=1.94&r2=1.95)