Adopt `Sendable` incrementally

[dnadoba]

Incremental Sendable adoption. Follow up of https://github.com/apple/swift-nio-ssl/pull/392

Properties are not marked with @preconcurrency because we otherwise get warnings because of https://github.com/apple/swift/issues/60508 I think that the latest Swift 5.7 toolchain doesn't produces any errors related to Sendable requirements in this context and we therefore don't actually need @preconcurrency here.

[Lukasa]

Can you also clean up the test errors on nightly?