Don't just rely on the CallbackHandler classname as cache key for remoting connections

The commit here fixes the same issue that was fixed in EJBCLIENT project https://github.com/jbossas/jboss-ejb-client/pull/48 where the CallbackHandler class name was used as a part of the cache key for maintaining remoting connections. Instead, this fix ensures that the CallbackHandler instance is taken into account for the cache key.