This removes SocketAsyncEngine.Token. The token ensured each socket
has a unique key for receiving events on a poll thread.

These unique tokens were introduced to deal with fd recyling leading
to false events being delivered to the new socket (when the fd is used
as the key).

This is not an issue for most operations because they can be retried.
It is an issue specifically for the connect operation which is an
on-going operation for which the status must be read using SO_ERROR
socket option when the socket becomes writable.

The connect-case is solved by checking the socket is writable
before reading the SO_ERROR socket option.

By using the fd as handles we can remove the logic for generated
unique tokens, which includes spinning up additional poll threads
in case unique handles are exhausted.

…ails