[inetstack] Move UDP stack to objects

[iyzhang]

This PR moves the UDP stack towards our new architecture. We have moved functionality into each SharedUDPQueue, including a per-queue background coroutine that sends pending packets. Since most of the time we will be able to send synchronously, it's better to avoid the lookup on each send and just rely on the scheduler to wake the appropriate background coroutine. I also added a clean up for the background coroutine on socket close.

This PR has a few lingering issues:

1. I moved the UDP queues to the AsyncQueue data structure, which is currently unbounded. We should move it back to being bounded and possibly using the mpsc abstractions from the futures crate. We should decide what kind of blocking behavior we want when the receive or send queues fill up.
2. I added a SharedAsyncQueue data structure but wound up not using it. I left it in case we need it somewhere else.

Other remaining tasks:

1. Add UDP support to the socket state machine and have the SharedUdpQueue check it on each operation.
2. Add pending ops to the SharedUdpQueue and cancel them when the queue is closed.