sorted/sqlkv: support for adding optional 'db.' prefix before table names

This is so configurations using the same database server for many
databases (multiple queues + the index) can all share the same
underlying *sql.DB

Change-Id: Iae7d4835fe9045008709c953e36b729729b1b6f4
sorted/mysql: don't put database name in the DSN, share connections

Change-Id: Ia4d399e72a1290f68dc2b0de71945176c6aac482