proposal: Set effective_cache_size to greater of .conf value, shared_buffers

Reference: http://postgresql.1045698.n5.nabble.com/Simple-join-doesn-t-use-index-td5738689.html

This is a pretty common gotcha: user sets shared_buffers but misses
the esoteric but important effective_cache_size. ISTM
effective_cache_size should always be >= shared buffers -- this is a
soft configuration error that could be reported as a warning and
perhaps overridden on the fly.

merlin