Fix HTTP/2 session receive window handling for small sizes (#9117)

[maskit]

Back porting #9117 to 9.2.x. I had to make server_rwnd_is_shrinking public member to resolve conflicts.

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This fixes the way we handle session window sizes if the user configures proxy.config.http2.initial_window_size_in to be smaller than the HTTP/2 protocol's default value of 65,535. For streams, we can communicate to the client a smaller stream size via a SETTINGS frame, but not so for smaller session window sizes. The way HTTP/2 hosts are expected to reduce their receive session window size is by receiving DATA frames without replying with WINDOW_UPDATE frames until the window falls to the desired value. ATS already handled this latter behavior correctly (letting the window size fall without sending WINDOW_UPDATE frames for the session until it reached the desired smaller value). However, it incorrectly reported an error and sent a GOAWAY frame when the user sent DATA content above the reduced window size before ATS gave the client a chance to reduce it via DATA frames. This fixes the receive session window check to allow for a shrinking session window.

Fixes #9115

(cherry picked from commit 4fcb9b4)

Conflicts:
proxy/http2/Http2ConnectionState.cc

[zwoop]

[approve ci autest]

[ezelkow1]

[approve ci autest]

[bneradt]

Looks like the s3_auth test needs to be updated in 9.2.x:

Generating Report: --------------
1 Tests were skipped:
--------------------------------------------------------------------------------
 Test "conn_timeout" Skipped
    File: conn_timeout.test.py
    Directory: /home/jenkins/workspace/Github_Builds/autest/src/tests/gold_tests/timeout
  Reason: Test requires privilege

 Test: s3_auth_config: Exception
    File: s3_auth_config.test.py
    Directory: /home/jenkins/workspace/Github_Builds/autest/src/tests/gold_tests/pluginTest/s3_auth
     Reason: Traceback (most recent call last):
       File "/home/jenkins/.local/share/virtualenvs/tests-iiJvYDvR/lib/python3.8/site-packages/autest/core/runtesttask.py", line 34, in __call__
         tl = self.__logic.Run(self.__test)
       File "/home/jenkins/.local/share/virtualenvs/tests-iiJvYDvR/lib/python3.8/site-packages/autest/runlogic/runlogic.py", line 18, in Run
         if not tmp.Start(obj):
       File "/home/jenkins/.local/share/virtualenvs/tests-iiJvYDvR/lib/python3.8/site-packages/autest/runlogic/test.py", line 90, in Start
         loadTest(self.__test)
       File "/home/jenkins/.local/share/virtualenvs/tests-iiJvYDvR/lib/python3.8/site-packages/autest/core/test.py", line 292, in loadTest
         execFile(fileName, locals, locals)
       File "/home/jenkins/.local/share/virtualenvs/tests-iiJvYDvR/lib/python3.8/site-packages/autest/common/execfile.py", line 16, in execFile
         exec(safeCompile(f.read(), fname), globals, locals)
       File "/home/jenkins/workspace/Github_Builds/autest/src/tests/gold_tests/pluginTest/s3_auth/s3_auth_config.test.py", line 65, in <module>
         ts.Disk.traffic_out.Content = "gold/s3_auth_parsing_ts.gold"
     AttributeError: 'Disk' object has no attribute 'traffic_out'

I can create a 9.2.x PR to fix that. Then hopefully the tests should work better.