Added signal escalation to command executor.

This patch adds a two step signal escalation: 1) SIGTERM is sent to
the process tree 2) If the process tree root hasn't been reaped within
escalationTimeout (3 seconds), SIGKILL is sent to the process tree.
Currently, this may leave behind orphan processes if parent processes
terminates and leave non-responsive processes behind. This will be
captured when PID namespaces are in place, as the orphan processes
will end up hanging off the process tree root instead of init.

Review: https://reviews.apache.org/r/19239