Get rid of tor-proto's ChannelImpl, and use the reactor more instead

Instead of awkwardly sharing the internals of a tor-proto Channel between the reactor task and any other tasks, move most of the internals into the reactor and have other tasks communicate with the reactor via message-passing to allocate circuits and send cells.

This makes a lot of things simple, and has convenient properties like not needing to wrap the Channel in an Arc (though some places in the code still do this for now).

A lot of test code required tweaking in order to deal with the refactor; in fact, fixing the tests probably took longer than writing the mainline code (!). Importantly, we now use tokio's tokio::test annotation instead of async_test, so that we can run things in the background (which is required to have reactors running for the circuit tests).

This is an instance of #205 (closed), and also kind of #217 (closed).