Rename Connection::Close() to Depool().

This makes much more sense, and cleans up the @todo, but I'll experiment with
RAII in the next commit or so.
Clean up UvUpdateTimerCallback.

IoCore::End() is static now, so we can call it here.
Use this->tcp in Connection::Read.

This will always be the same as the old stream parameter, bugs aside.
Rework and comment IO control structures.
Close TCP connections at destruction time.

This means Connection is sort-of-RAII-ish now!
Improve network error handling.

This squishes out the last few todos, I think.