pgsql: bufmgr: Add some more error checking [infrastructure] around pin

bufmgr: Add some more error checking [infrastructure] around pinning

This adds a few more assertions against a buffer being local in places we
don't expect, and extracts the check for a buffer being pinned exactly once
from LockBufferForCleanup() into its own function. Later commits will use this
function.

Reviewed-by: Heikki Linnakangas <hlinnaka(at)iki(dot)fi>
Reviewed-by: Melanie Plageman <melanieplageman(at)gmail(dot)com>
Discussion: http://postgr.es/m/419312fd-9255-078c-c3e3-f0525f911d7f@iki.fi

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/819b69a81d307e2ad8e9752cf062ebb57813960d

Modified Files
--------------
src/backend/storage/buffer/bufmgr.c | 44 +++++++++++++++++++++++++++----------
src/include/storage/bufmgr.h | 1 +
2 files changed, 33 insertions(+), 12 deletions(-)