Add watchdog for Heka services

This patch adds a new 'heka.sandbox.watchdog' type of Heka message that
is used by Sensu to check the availability of a service. In the current
implementation the message is issued by a filter for all Heka services
and the check is sent locally to the Sensu client.

Change-Id: I6b3c9d808dcec7d8c15c442390dfbda2032f38ce
9 files changed