Ability to cancel queued but unsent websocket messages

It would be useful to be able to cancel websocket messages that are pending on a connection but aren't yet sent (or in the process of being sent).

The use case is a slow websocket consumer that isn't accepting any more data. The generation of new messages can be paused based on the existing callbacks, messages that are already in the AbstractFramedChannel's pendingFrames collection can't be revoked