Cleanly exit a forked executor if the slave terminates during launch.

Review: https://reviews.apache.org/r/22481
Fixed a flaky test: SlaveTest.TerminatingSlaveDoesNotReregister.

Review: https://reviews.apache.org/r/22472
Fixed ZooKeeperProcess and ProcessWatcher to avoid deadlock during ZooKeeperProcess termination.

Review: https://reviews.apache.org/r/22521