Proper locking for datanode leader election

[todvora]

The current leader election logic in the datanode doesn't do any locking, so it may happen that we have two leaders in the same time. So far it won't cause any problems, but we should fix this soon.

[bernd]

We should investigate whether we can avoid needing a Data Node leader node. The tasks currently only executed on the leader node might be candidates for the job scheduler.