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
