Simple LRU for clog and (later) subtrans

This patch extracts page buffer pooling and the simple
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.

Servus
Manfred