Fixed a flaky test.

Review: https://reviews.apache.org/r/23543
Added a low level scheduler example using pthread.

Review: https://reviews.apache.org/r/23216
Changed healthy status to be sent when task killed after health check.

Currently after the health check receives configured amount of task
fails it will initiate a kill task. However currently the task kill
status update doesn't set the healthy field so it's unclear was it
killed because health check or other reasons. This patch sets the health
check field when the task is killed because of failing health checks.

Review: https://reviews.apache.org/r/23520
Fixed variable naming in a test.
Support todo and namespace cpp style and fix violations