Made the handeling of the join request batch oriented.

In large clusters when a new elected master is chosen, there are many join requests to handle. By batching them up the the cluster state doesn't get published for each individual join request, but many handled at the same time, which results into a single new cluster state which ends up be published.

This PR is for the improve zen branch.
while we are at it, can we move this log to debug? its very noisy and it can happen with join retry logic