Correctly destroy a backing off connection

[adam-fowler]

If a connection pool has multiple requests backing off, when we are in the connectionCreationFailing state only one of these is allowed to continue. All others are destroyed. Previously we called destroyFailedConnection but this would precondition for multiple reasons. Instead we should have a function specific for destroying a backoff connection, which also clears the timers correctly.

Correctly destroy a backing off connection

[codecov]

Codecov Report

❌ Patch coverage is 69.23077% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.53%. Comparing base (88e1545) to head (54f98ea).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...nPoolModule/PoolStateMachine+ConnectionGroup.swift	77.77%	2 Missing ⚠️
...ources/ConnectionPoolModule/PoolStateMachine.swift	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #620      +/-   ##
==========================================
+ Coverage   76.37%   76.53%   +0.15%     
==========================================
  Files         134      134              
  Lines       10170    10179       +9     
==========================================
+ Hits         7767     7790      +23     
+ Misses       2403     2389      -14     

Files with missing lines	Coverage Δ
...nPoolModule/PoolStateMachine+ConnectionGroup.swift	90.21% <77.77%> (+0.50%)	⬆️
...ources/ConnectionPoolModule/PoolStateMachine.swift	90.12% <50.00%> (+0.63%)	⬆️

... and 2 files with indirect coverage changes

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[fabianfett]

Thank you!