Add support for running multiple workers for each secondary instance.

Note this requires updating the secondary watcher config for each instance to specify a
numWorkers setting before the new code will run.

NOTE: DO NOT MERGE this yet.  Once I get feedback then I will update the checked in dev conf and the chef configs before merging this.  Unfortunately I don't see a clean way to set this up with a default because it isn't in reference.conf since it is config that is specific to each secondary but read by the secondarywatcher.