Driver validation during execute_statement delays library path validation

[jsvd]

The plugin only loads the driver(s) during open_jdbc_connection, that is executed with execute_statement.

Because of this, if the file passed to jdbc_driver_library doesn't exist, an error only happens when the scheduler triggers for the first time.

Also, the execute_statement adquires the connection_lock but if the open_jdbc_connection raises an exception the lock isn't released, causing logstash to deadlock during shutdown in close_jdbc_connection