Fixed update puller unpausing during switch to slave and concurrent master election

It could so happen that masterIsAvailable event, which pauses update puller, is fired during switch to slave is in progress. This could lead to a situation when update puller was unpaused by slave switch and then immediately paused by masterIsAvailable. Such race left update puller paused until next election.
This PR adds unpausing of update puller when cluster member sees itself as available slave.