Use simplehash for backend-private buffer pin refcounts.

Use simplehash for backend-private buffer pin refcounts.

Replace dynahash with simplehash for the per-backend PrivateRefCountHash
overflow table.  Simplehash generates inlined, open-addressed lookup
code, avoiding the per-call overhead of dynahash that becomes noticeable
when many buffers are pinned with a CPU-bound workload.

Motivated by testing of the index prefetching patch, which pins many
more buffers concurrently than typical index scans.

Author: Peter Geoghegan <pg@bowt.ie>
Suggested-by: Andres Freund <andres@anarazel.de>
Reviewed-By: Tomas Vondra <tomas@vondra.me>
Reviewed-By: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CAH2-Wz=g=JTSyDB4UtB5su2ZcvsS7VbP+ZMvvaG6ABoCb+s8Lw@mail.gmail.com