Cleanup Http2StreamHandling vs. Http2Multiplexer

[jrudolph]

There have been some attempts to clean up the interface between stream handling and multiplexer but some issues still remain:

• The call path of Sending.pullNextFrame -> OutStream.nextFrame / endStreamIfPossible is more complex than necessary.
• A stream might enqueue itself multiple times because the above call path makes it harder to keep track in OutStream whether a stream is currently enqueued or not.
• We needed to introduce NothingToSend to deal with duplicate entries in the sendableOutStream queue in #3873. This can be removed once we can ensure that no streams are enqueued multiple times.