[WFLY-3316] Multi threaded ejb invocations via remote-naming produce EJBCLIENT000025 if the Context is closed

Adding the EJBClientHandler to the CacheKey for avoiding mixing
EJBClientContext among threads if the flag "jboss.naming.client.ejb.context" in the properties is set to "true"

BZ: https://bugzilla.redhat.com/show_bug.cgi?id=1094380
Jira: https://issues.jboss.org/browse/WFLY-3316