Fix problem with duplicate change suppression.
Fix problem with duplicate change suppression.

Jobs for changes weren't being launched for DependentQueueManagers
because its addChange method called the superclass's addChange,
which had the same duplicate suppresion logic.  This breaks
out the bulk of that into a private method that can be called
by both classes.

Change-Id: Ib47be82f255bab38ce03c7af7fb5b700e1956aaa

wikimedia/integration-zuul,devdattakulkarni/zuul_messaging,gooddata/zuul,gooddata/zuul,gooddata/zuul,gooddata/zuul,bhuvan/zuul,devdattakulkarni/zuul_messaging,wikimedia/integration-zuul,wikimedia/integration-zuul,devdattakulkarni/zuul_messaging,gooddata/zuul,wikimedia/integration-zuul