Add a emptyQueue method to Redis event bus emitter

[thibaultponcelet]

We use this in our test suite in order to empty event bus queues without removing the queue name set. Usefull when different processes are using reporters and are not reloaded for each scenario

[jbpros]

Closed by 26f0ad912846abd97754e6d5ca2f09a11e147c3c and 6f77e36d3c3fff4b0ec78e01773660faef0f3af4