NIOLock and NIOLockedValueBox are too strict

[Lukasa]

NIOLock is particularly scary, but NIOLockedValueBox has some limitations too that need to be tidied up.

The following functions are dangerous:

https://github.com/apple/swift-nio/blob/9ff5fddb3d4da3b62dadb8ff83944acf6b0eabf9/Sources/NIOConcurrencyHelpers/NIOLock.swift#L232-L254

Here, withLock needs to take its closure sending and the return value needs to be sending too. In Swift 5, these both need to be Sendable, with an unchecked version available.

https://github.com/apple/swift-nio/blob/9ff5fddb3d4da3b62dadb8ff83944acf6b0eabf9/Sources/NIOConcurrencyHelpers/NIOLockedValueBox.swift#L35-L39

Here, withLockedValue needs again to take its closure sending. Optionally, we could relax its Sendable requirement, and also return the value sending, but given our need to remain compatible with Swift 5 we should probably leave it stricter for now. In Swift 5, the closure probably needs to be @Sendable.