least-recently-used strategy from clog.c into slru.c.  It doesn't
change any visible behaviour and passes all regression tests plus a
TruncateCLOG test done manually.

Apart from refactoring I made a little change to SlruRecentlyUsed,
formerly ClogRecentlyUsed:  It now skips incrementing lru_counts, if
slotno is already the LRU slot, thus saving a few CPU cycles.  To make
this work, lru_counts are initialised to 1 in SimpleLruInit.

SimpleLru will be used by pg_subtrans (part of the nested transactions
project), so the main purpose of this patch is to avoid future code
duplication.

Manfred Koizar