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

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.