flushEventQueue
Flushes any queued events from the system when it is considered safe to do so by any Barriers that may be blocking.
This method will not override those Barrier implementations whose Barrier.isFlushable returns false. But when non-flushable barriers open, a flush will still occur.
Return
A Single onto which you can subscribe to receive the completion with the eventual error. The returned Single, in case of success, completes when the flush request is accepted, not when all the events have been flushed.