WFLY-1514 Prevent NPE in InfinispanSessionManager's cache notication handling methods

https://issues.jboss.org/browse/WFLY-1514

The commit here fixes the NPE's that InifinispanSessionManager runs into in its cache notification event handler methods due to race conditions. This causes intermittent failures in certain testcases as noted in the linked JIRA.

Please do not merge this till Paul F approves this.