Improve management of SLRU statistics collection.

Instead of re-identifying which statistics bucket to use for a given
SLRU on every counter increment, do it once during shmem initialization.
This saves a fair number of cycles, and there's no real cost because
we could not have a bucket assignment that varies over time or across
backends anyway.

Also, get rid of the ill-considered decision to let pgstat.c pry
directly into SLRU's shared state; it's cleaner just to have slru.c
pass the stats bucket number.

In consequence of these changes, there's no longer any need to store
an SLRU's LWLock tranche info in shared memory, so get rid of that,
making this a net reduction in shmem consumption.  (That partly
reverts fe702a7b3.)

This is basically code review for 28cac71bd, so I also cleaned up
some comments, removed a dangling extern declaration, fixed some
things that should be static and/or const, etc.

Discussion: https://postgr.es/m/3618.1589313035@sss.pgh.pa.us

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