In `clear_queue` we drop all the queued frames for a stream, but this doesn't
take into account a buffered frame inside of the `FramedWrite`. This can lead
to a panic when `reclaim_frame` tries to recover a frame onto a stream that has
already been destroyed, or in general cause wrong behaviour.

Instead, let's keep track of what frame is currently in-flight; then, when we
`clear_queue` a stream with an in-flight data frame, mark the frame to be
dropped instead of reclaimed.