WIP: Make NIOFileDescriptor/FileRegion/IOData Sendable

[weissi]

• this is currently slow, ugly and overly complex
• there's much potential for optimisation

Ideally, we'd duplicate NIOFileHandle so we have two versions

1. roughly the one implemented in this PR which is Sendable
2. a second, internal one just for the selectors/base channels, not Sendable, no atomics (exactly like the old one)

But I'm not sure that works. This PR is here to derisk having to keep one NIOFileHandle for both.

[FranzBusch]

Thanks for opening the discussion here. So far we have stayed away from making those changes and debated if we should just drop the whole IOData support in a potential NIO 3. Though we haven't come to any agreement on that matter yet. This would definitely solve the immediate pain of the Sendable problem that we are facing now.

[FranzBusch]

After having looked at the strict checking this is one of the last remaining issues in NIOCore. The PR itself looks good and IMO we should move forward with this.