Handle a change in the master node. Doesn't matter whether this was called
from a nodeCreated or nodeDeleted event because there are no guarantees
that the current state of the master node matches the event at the time of
our next ZK request.
Uses the watchAndCheckExists method which watches the master address node
regardless of whether it exists or not. If it does exist (there is an
active master), it returns true. Otherwise it returns false.
A watcher is set which guarantees that this method will get called again if
there is another change in the master node.