Allow caching query information

[fishcakez]

It's possible to prepare a query once, and execute the query on any process in the pool. This can save a round trip to the database. Please see https://github.com/fishcakez/postgrex_cache for an example - there should be a better API to handle this in the next release of postgrex. This will still work with pgbouncer etc if prepare: :unnamed is passed to Postgrex.start_link/child_spec but has limited benefit (encodes params before checking out connection) because the query will always be prepared and executed.