fix remaining warnings & enable -warnings-as-errors in CI

[weissi]

Motivation:

Warnings are annoying.

Modifications:

• Remove unnecessary use of Foundation.Thread which isn't Sendable.
• Remove now unnecessary @retroactives.
• Enable -warnings-as-errors in CI

Result:

• Warnings can't sneak in as easily anymore
• No more warnings left in swift-nio

$ rm -rf .build/arm64-apple-macosx/ && swift build --build-tests -Xswiftc -warnings-as-errors > /dev/null
echo $?
$ echo $?
0

[Lukasa]

Thanks! Can you tweak the CI config to re-enable warnings as errors and lock this win in?