Automatic merge from submit-queue

WatcherDispatcher for federated controller tests

This PR fixes a problem when 2 informers create a watch for the same resource using the same client. Previously only one informer would receive updates.

cc: @quinton-hoole @wojtek-t @kubernetes/sig-cluster-federation

quinton-hoole: To provide some more context to those doing cherrypicking, the bug that this PR fixes makes federated unit tests fail intermittently, and generally behave very poorly.