core: replaced SkipHistory exception by a key in mess_data: SkipHistory was skipping all remaining triggers just to skip history, which is not the intented behaviour. Now history can be skipped by setting mess_data[u'history'] = C.HISTORY_SKIP, this way we won't skip importants triggers. When history is skipped, mess_data[u'extra'][u'history'] will be set to C.HISTORY_SKIP for frontends, so they can inform user that the message is not stored locally.