Tweak flow control

[iyangsj]

• Tweak initialization of flow control
• Tweak API for flow control configurations
• Add more comments about flow control
• Fix the unit test client_send_multi_requests_blocked_by_conn_flow_control

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 90.69767% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 94.98%. Comparing base (f449431) to head (141e92d).

Files	Patch %	Lines
src/connection/stream.rs	71.42%	4 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #275 +/- ## =========================================== - Coverage 95.38% 94.98% -0.40% =========================================== Files 50 50 Lines 33483 33474 -9 =========================================== - Hits 31937 31795 -142 - Misses 1546 1679 +133 ```

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