Send an unregister message when the slave is shutting itself down.

Review: https://reviews.apache.org/r/22367
Added metrics for port mapping network isolator.

Review: https://reviews.apache.org/r/23492
Fix executor passing STDIN pipe to health check.

Previous fix for STDIN only fixes the health check command not passing
in STDIN pipe by reading file STDIN. However the health check command
is still getting STDIN piped as the command executor is creating a
PIPE to STDIN when launching health check. To really fix this we need
to also fix not passing anything to STDIN when launching health check.

Review: https://reviews.apache.org/r/23518
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