Simplify PollingMonitor by exposing a public way to fake polling.

Change-Id: I20c40e4966723d20160f38c14ae779e7d10a3738
Properly treat the heartbeats duration from workers to be in milliseconds.

We didn't properly convert the heartbeat into milliseconds so we caused
the expiration of workers to take roughly 1000 times longer than expected.

Change-Id: Iedde718b7ea943844d6e085b22b5ab0b16703c82