refactor Queue.hasFlush: Boolean to Queue.flush: Option[Bool]. (#2245)

* refactor Queue.hasFlush: Boolean to Queue.flush: Option[Bool].

Using factory Queue(..., hasFlush = true) won't take effects, since in the Queue.apply API, Queue Module is not exposed, thus even user defines hasFlush = true, there is no place for them to give the flush signal.
This commit fix this, refactor Queue.hasFlush: Boolean to Queue.flush: Option[Bool], makes user be able to pass the flush signal into Queue Module.

* use zip to connect.

* refactor docs.

Co-authored-by: Megan Wachs <megan@sifive.com>