Simplify leader election / leader session ID assignment

Currently, there are two separate actions and znodes involved in leader election and communication of the leader session ID and leader URL.

This leads to some quite elaborate code that tries to make sure that the leader session ID and leader URL always eventually converge to those of the leader.

It is simpler to just encode both the ID and the URL into an id-string that is attached to the leader latch znode. One would have to create a new leader latch each time a contender re-applies for leadership.