Fix the websocket shutdown behavior

[TingDaoK]

The bug was introduced in PR #474

• is_writing_stopped = true shouldn't be set directly, there's a helper function s_stop_writing() that ensures subsequent calls to aws_websocket_send_frame() will fail.

Let's take a whole new approach these channel-shutdown-window-deadlock issues:

• add s_stop_reading_and_dont_block_shutdown() function that sets is_reading_stopped = true, but also increments the read window so that channel shutdown won't deadlock.
  • Most places that were setting is_reading_stopped = true now use this helper instead
• Revamp how aws_channel_shutdown() is called. Lots of channel behavior has changed since this websocket code was written.
  • If on the channel-thread, just call aws_channel_shutdown()
    • now that aws_channel_shutdown() is always async, we don't need to defensively schedule a task to call it
  • If off-thread, use s_schedule_channel_shutdown_from_offthead()
    • now that this is only called from aws_websocket_close(), or when the refcount goes to zero, we can assume the user is OK if reading stops, and it can call s_stop_reading_and_dont_block_shutdown() on the way to shutting down.
• Add the test to verify that send after close should fail

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 90.90909% with 2 lines in your changes missing coverage. Please review.

Project coverage is 79.61%. Comparing base (7db2452) to head (791c582).

Files	Patch %	Lines
source/websocket.c	90.90%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #483 +/- ## ========================================== + Coverage 79.55% 79.61% +0.06% ========================================== Files 27 27 Lines 11688 11683 -5 ========================================== + Hits 9298 9302 +4 + Misses 2390 2381 -9 ```

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