Make local buffers pin limit more conservative

Make local buffers pin limit more conservative

GetLocalPinLimit() and GetAdditionalLocalPinLimit(), currently in use
only by the read stream, previously allowed a backend to pin all
num_temp_buffers local buffers. This meant that the read stream could
use every available local buffer for read-ahead, leaving none for other
concurrent pin-holders like other read streams and related buffers like
the visibility map buffer needed during on-access pruning.

This became more noticeable since b46e1e54d07, which allows on-access
pruning to set the visibility map, which meant that some scans also
needed to pin a page of the VM. It caused a test in
src/test/regress/sql/temp.sql to fail in some cases.

Cap the local pin limit to num_temp_buffers / 4, providing some
headroom. This doesn't guarantee that all needed pins will be available
— for example, a backend can still open more cursors than there are
buffers — but it makes it less likely that read-ahead will exhaust the
pool.

Note that these functions are not limited by definition to use in the
read stream; however, this cap should be appropriate in other contexts.

Reported-by: Alexander Lakhin <exclusion@gmail.com>
Author: Melanie Plageman <melanieplageman@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/97529f5a-ec10-46b1-ab50-4653126c6889%40gmail.com