Assert slave not master.

Test asserting on master that has to be slave.

Review: https://reviews.apache.org/r/22779
Fixed a compiler error due to ambiguous UNREACHABLE in Subprocess.
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
Address ben review comments