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

src/include/storage/bufmgr.h

@@ -134,6 +134,7 @@ extern void ReleaseBuffer(Buffer buffer);
 extern void UnlockReleaseBuffer(Buffer buffer);
 extern void MarkBufferDirty(Buffer buffer);
 extern void IncrBufferRefCount(Buffer buffer);
+extern void CheckBufferIsPinnedOnce(Buffer buffer);
 extern Buffer ReleaseAndReadBuffer(Buffer buffer, Relation relation,
 								   BlockNumber blockNum);