fix almost all Sendable warnings

[weissi]

Motivation:

Opening the swift-nio repository made me warning blind because there were always so many trivially fixable warnings about things that were correct but cannot be understood by the compiler.

Modifications:

Fix all the sendable warnings that popped up, except for one test where NIOLockedValueBox<Thread?> isn't sendable because Foundation.Thread seemingly isn't Sendable which is odd. Guessing that'll be fixed on their end.

Result:

• Fewer warnings
• Less warning-blindness
• More checks

[dnadoba]

And that without any unsafe/unchecked swift! Nicely done!

[Lukasa]

The intention here has been to wait until we could use the isolated views on ELP and ELF to do this work, to minimise the amount of runtime isolation checking. As the principal advocate for avoiding preconditionOnEventLoop, would you prefer to silence these warnings at that cost?

[Lukasa]

That comment applies to the changes in the production code, naturally: the changes in the tests are going to be just fine either way.

[weissi]

The intention here has been to wait until we could use the isolated views on ELP and ELF to do this work, to minimise the amount of runtime isolation checking. As the principal advocate for avoiding preconditionOnEventLoop, would you prefer to silence these warnings at that cost?

@Lukasa I really would want to see these warnings go away because there are just too many of them, I lose track of others (point in case this one: #2995 (comment)).

I hear you w.r.t. to the preconditionInEventLoop but here's why I think we should still do it: At first, I only wanted to do tests & the upgraders (because they're not super perf critical). But then I noticed that's like 99% of all of them. None of the code I changed is in actually perf-critical code or changes any of the properties of the code.

And once we have the isolated views (which are fab btw), we can still replace all the code, it's super easy to spot by grepping for NIOLoopBound, right?

tl;dr: This improves things and I mandate that it doesn't regress anything. So I don't think there's any point in waiting.