U1db implementation

This is the method that listens for events from the application logic. Say, the client hits the "Sync" button then it should preferably set some variable to true, and the `check_new_sync` will detect it and attempt a new sync. I did this keeping the `poller` in mind. It works like this:

* Poll to see incoming updates on SUB socket. (Mass updates. We are not using this now.)
* Poll to see incoming replies on REQ socket.
* If above two are not true, it `check_new_sync`.