Refactored health check subsystem.

- Moved task status update logic to
  MarathonHealthCheckManager, refactored to
  delegate state to the HealthCheckActor
- Renamed DelegatingHealthCheckManagerTest to
  MarathonHealthCheckManagerTest
- Added EventFilters to health check unit tests.
- Deleted DelegatingHealthCheckManager
- Dropped the --executor_health_checks flag
- Removed --executor_health_checks from flags doc
- Updated the health checks doc
- Updated the REST doc
- Updated bridge net example to include cmd check
- Fixes #664
- Fixes #724
No.  The limitation is that there may be at most one COMMAND health check.  Will update this text.