Lazy-attach/re-attach the handles.

[Berrysoft]

Currently the file handles are attached immediately after created. That makes it impossible to write a multi-threading web server: listen in one thread, and spawn the accepted socket to other threads. The solution is:

• Make it attach lazily. Maybe at the first submission.
• Make it be aware of thread sending. If the thread id is changed, it should automatically attach itself to the current runtime.

[Berrysoft]

/cc @bdbai

[bdbai]

Without incurring runtime cost, I think we can mark all Sockets/FDs !Send. Users may manually detach an object, obtaining a Send, type-stamped wrapper to send to another thread. To get back the original wrapped object, users will have to call attach on the wrapper. Does it make sense?

[Berrysoft]

Well, although the driver attached is thread local, the handle itself is technically thread safe...

[Berrysoft]

I've also decided to not use undocumented APIs, which means I cannot detach a handle from the driver. A little painful.