arjdbc: performance improvement in setup_jndi_factory

By implementing `JdbcConnectionFactory` using the class & mixin pattern instead of using a block, we saw some great performance improvement under high load in Kill Bill (http://github.com/killbill/killbill).

The stack is probably too complex to share details on our benchmark, but https://github.com/jruby/jruby/issues/1401 has related numbers.