Declare ThreadSpecificVariable public (#102)
Declare ThreadSpecificVariable public (#102)

Motivation:

It's often useful for users to be able to store a thread-specific value (and so also EventLoop specific). We have ThreadSpecificVariable for this which was internal.

Modifications:

Change ThreadSpecificVariable to be public.

Result:

Fixes [#61].
apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio