Connection shutdown with buffered data

[TingDaoK]

Issue #, if available:

• Another similar fix to https://github.com/awslabs/aws-c-io/pull/650 and https://github.com/awslabs/aws-c-io/pull/642
• We have buffer in HTTP connection, and connection had different window than the stream and it already read to the buffer, even if the stream has 0 window.
• So, when we read the last bytes into connection buffer, and channel starts to shutdown, the buffered data will be unread and stream will fail, while it should wait until those data to be read.

Description of changes:

• very similar fix to https://github.com/awslabs/aws-c-io/pull/650, put the channel shutdown on hold until the buffer data read from downstream.
• Also a TODO from https://github.com/awslabs/aws-c-http/pull/474, which we thought it only affects websocket, as it's the only downstream for HTTP/1.1 connection, however, since the difference of the window for connection and stream, it affects the HTTP as well. :(

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 97.14286% with 1 line in your changes missing coverage. Please review.

Project coverage is 79.45%. Comparing base (b5684c7) to head (c9cfa04).

Files	Patch %	Lines
source/h1_connection.c	97.14%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #482 +/- ## ========================================== - Coverage 79.51% 79.45% -0.07% ========================================== Files 27 27 Lines 11661 11689 +28 ========================================== + Hits 9272 9287 +15 - Misses 2389 2402 +13 ```

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