db/def*, db/query, db/execute! should cache their PreparedStatement

[coconutpalm]

Create an internal sql-fn API that accepts the ::SQL-FN, current connection, and PreparedStatement and returns a closure over atoms containing the connection and PreparedStatement. When this function is executed it should reuse the PreparedStatement from prior invocations if the connection is still the same.

Then update defquery, defstatement, query, and execute! to use this helper.