Allow accessing send queue count from WebSocketConnection

[Shane32]

Allows derived classes to check the send queue count and override SendMessageAsync in case the number of messages in the outbound queue is growing out of control. Perhaps in the future backpressure options will be available as a built-in feature, but with #1154 it should not be necessary.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.79%. Comparing base (a84c553) to head (9d25258).
⚠️ Report is 43 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1156      +/-   ##
==========================================
- Coverage   90.88%   90.79%   -0.10%     
==========================================
  Files          50       50              
  Lines        2425     2433       +8     
  Branches      432      433       +1     
==========================================
+ Hits         2204     2209       +5     
- Misses        175      176       +1     
- Partials       46       48       +2     

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