Tweak buffer manager so that 'internal' accesses to a buffer do not

Tweak buffer manager so that 'internal' accesses to a buffer do not
advance its usage_count.  This includes writes of dirty buffers triggered
by bgwriter, checkpoint, or FlushRelationBuffers, as well as various
corner cases that really ought not count as accesses to the page.
Should make for some marginal improvement in the quality of our decisions
about when to recycle buffers.  Per suggestion from ITAGAKI Takahiro.