Use prepare/execute for queries and remove handle_query

A query should be carried out as a prepare and a execute so that parameter encoding does not have to be done inside handle_query where an error will crash the connection. It also means adapters do not have to handle the prepare/encode/execute logic themselves.

However this leads to an issue where un-named queries won't be closed after a query without adding a round trip to explicitly close with handle_close. One way to solve this is to introduce a handle_execute_close and execute_close that allows an adapter to handle the execute and close in an efficient manor. The default implementation provided by __using__ would call handle_execute and handle_close.

elixir-ecto / db_connection

Use prepare/execute for queries and remove handle_query #3