Fix sendable warnings

[fabianfett]

As NIO becomes more and more Sendable clean, new issues arise in libraries using NIO. This pr fixes some new ones.

[codecov]

Codecov Report

Attention: Patch coverage is 65.71429% with 12 lines in your changes missing coverage. Please review.

Project coverage is 61.75%. Comparing base (7c29718) to head (ab0695f).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...urces/ConnectionPoolModule/NIOLockedValueBox.swift	10.00%	9 Missing ⚠️
Sources/ConnectionPoolModule/NIOLock.swift	88.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #533      +/-   ##
==========================================
- Coverage   61.78%   61.75%   -0.04%     
==========================================
  Files         125      125              
  Lines       10074    10089      +15     
==========================================
+ Hits         6224     6230       +6     
- Misses       3850     3859       +9     

Files with missing lines	Coverage Δ
Sources/PostgresNIO/New/PSQLTask.swift	77.77% <ø> (ø)
Sources/PostgresNIO/PostgresDatabase+Query.swift	32.14% <ø> (ø)
Sources/ConnectionPoolModule/NIOLock.swift	74.46% <88.00%> (+1.74%)	⬆️
...urces/ConnectionPoolModule/NIOLockedValueBox.swift	30.76% <10.00%> (-69.24%)	⬇️