pgsql: Allow BufferAccessStrategy to limit pin count.

Allow BufferAccessStrategy to limit pin count.

While pinning extra buffers to look ahead, users of strategies are in
danger of using too many buffers.  For some strategies, that means
"escaping" from the ring, and in others it means forcing dirty data to
disk very frequently with associated WAL flushing.  Since external code
has no insight into any of that, allow individual strategy types to
expose a clamp that should be applied when deciding how many buffers to
pin at once.

Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Melanie Plageman <melanieplageman@gmail.com>
Discussion: https://postgr.es/m/CAAKRu_aJXnqsyZt6HwFLnxYEBgE17oypkxbKbT1t1geE_wvH2Q%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/3bd8439ed628c7e9ac250b1a042d9044303c37e7

Modified Files
--------------
src/backend/storage/aio/read_stream.c |  5 +++++
src/backend/storage/buffer/freelist.c | 42 +++++++++++++++++++++++++++++++++++
src/include/storage/bufmgr.h          |  1 +
3 files changed, 48 insertions(+)