Added ackAllMilestones api to PausableExecutor

+ This API simplifies tests, specifically their tearDown methods.
Rather than needing to call ackMilestone numerous times to clean up
at the end of the test, this method can be called to ensure that the
thread running the production code isn't blocked on the tests.

Change-Id: I60730c52491b315aee571f4ae158f62d34b0dbc2