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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/a367c433ad015b889ee296b07de00017819915c4

Modified Files
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src/backend/storage/buffer/bufmgr.c | 68 +++++++++++++++++++------------------
src/tools/pgindent/typedefs.list | 2 ++
2 files changed, 37 insertions(+), 33 deletions(-)