Two problems solved:

- Fix session fixation vulnerability
- Fix stale Hazelcast session issue

Session fixation vulnerability occurred with Spring Security MVC
 as it used isRequestedSessionIdValid method on HttpServletRequest
 interface which was not implemented before this commit.

Stale Hazelcast session issue occurs when a request comes in with a
 valid JSESSIONID and a hazelcast.sessionId that corresponds to
 another Hazelcast session. In this case, we just used the existing
 session and used the incoming JSESSIONID to find the corresponding
 hazelcast.sessionId and Hazelcast session. Now we let the incoming
 hazelcast.sessionId to override the hazelcast.sessionId that
 corresponds to the request's JSESSIONID.

Fix #47