Support separating session reporting from subjects reporting

[ayalash]

Report session started to backslash on 'before_session_start' and add the session subjects on 'session_start'.
Blocked by: getslash/backslash#351

Another option: we might want this behavior to be configurable (in case someone is using an old server version). We can add configuration for it.

[ayalash]

As discussed f2f with @vmalloc, we decided that session_start will call notify_session_start method only if hasn't started yet.