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
Fixed SlaveRecoveryTest.RestartBeforeContainerizerLaunch test.

Review: https://reviews.apache.org/r/22441
Removed TODO that was done.

Review: https://reviews.apache.org/r/22577
Initial import for health check
Fixed Protobuf to JSON serialization of optional fields with default values.

Previously, only optional fields which where explicitly set would be serialized regardless whether default values was present.
This patch adds those fields explicitly.

Review: https://reviews.apache.org/r/21246
Cleaned up JSON work
Snapshot of current health check program.
Add task health check with custom command support