Polilng is a very common means of seeking changes, so Buildbot supplies a
utility parent class to make it easier. A poller should subclass
buildbot.changes.base.PollingChangeSource, which is a subclass of
ChangeSource. This subclass implements the Service methods, and
causes the poll method to be called every self.pollInterval
seconds. This method should return a Deferred to signal its completion.
Aside from the service methods, the other concerns in the previous section apply here, too.