Enriched the Java Docs in the src/java files.

Review: https://reviews.apache.org/r/21080
Checked IP header length for IP packet filters.

Review: https://reviews.apache.org/r/23525
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
Reenable health check tests
Allow overridable launcher dir