Fix connection leak by releasing the obtained connection properly

[rdlf0]

This PR fixes a connection leak that happens when checking the driver version in order to determine if JDBC 4.2 APIs should be used or not by the Hibernate integrators.

The connection was obtained through obtainConnection(), but was not properly released, as invoking close() doesn't actually make the connection available in the connection pool. Calling releaseConnection() on the other side follows the correct process for releasing the resources and this way the connection pool stays balanced and there are no leaks. The closing of the connection is then handled by Hibernate.

A workaround for the existing connection leak is providing the jadira.usertype.useJdbc42Apis system property. Either value (true or false) will make the above check unnecessary and a connection will not be obtained.