WIP: buffer manager rewrite (take 2)

Second iteration of buffer manager rewrite.  This uses the idea of a
usage counter instead of just a recently-used flag bit.  I allowed the
counter to go up to 5, but some playing around with that value would
be interesting.  (Tweak BM_MAX_USAGE_COUNT in
src/include/storage/buf_internals.h, then recompile the files in
src/backend/storage/buffer/.)  Also there are more GUC variables now
for controlling the bgwriter.

            regards, tom lane