Fix closing pending frames

[turuslan]

When yamux connection is closing it should:

• Send pending frames.
• Send GoAway frame.
• Close underlying connection.

But test showed that pending frames from stream_receivers and GoAway frame were not sent.

Commits introducing this bug:

• 61003c5 replaced VecDeque with Option,
  which caused loss of pending frames,
  because only last pending frame was stored in Option
• 297e9eb reordered sending and collecting pending frames,
  trying to fix loss of pending frames.
• 2867280 reverted 61003c5 Option back to VecDeque,
  so all pending frames are stored again.
• However 297e9eb was not reverted,
  so pending frames are collected,
  but not sent.

This PR provides test to reproduce it, and reverts 297e9eb and af8f693 to fix it.

[jxs]

Thanks for this! Yeah I overlooked that DrainingStreamReceiver pushes frames back to the end of the queue.
Will release a patch version with this fix later on