Username should appear in each line of log

We need to use slf4j's MDC (mapped diagnostic context) in order to show user ID for each line of code we have.
The MDC should start in the very first listener/filter and user id should be put there. Then when response is sent the MDC should be removed.
Note, that MDC has to be cleared after each request so that we don't leak.

If a not-registered user is using forum, then we should put there: [anonymous-1324]. Note, that last 4 numbers is last 4 numbers of SESSIONID.