Shut down the driver's connection pool safely

[gwynne]

These changes are now available in 4.7.2

Specifically, don't use the soft-deprecated AsyncKit API that calls EventLoopFuture.wait() internally; use the one that at least does it with Dispatch (best we can do without changing Fluent's driver interface).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.38%. Comparing base (9e924b3) to head (91f6ee0).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #94   +/-   ##
=======================================
  Coverage   82.38%   82.38%           
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  Files           7        7           
  Lines         210      210           
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  Hits          173      173           
  Misses         37       37           

Files	Coverage Δ
...ources/FluentSQLiteDriver/FluentSQLiteDriver.swift	72.72% <100.00%> (ø)