tor-proto: Rewrite circuit reactor run_once() loop to use select!.

This branch contains multiple types of changes:

• code motion that makes the reactor impl a bit more readable
  • 88cfc896
  • 903e0cb0
  • aeb8d8ed
• API changes needed for the main run_once() refactoring
  • 98c951d9
• new types for implementing the run_once() changes
  • 2dadbfc8
  • 3ff6964d
• the main part of the branch is the Reactor impl refactoring in 2eb09f4f
• post-refactoring cleanups
  • 1aeaefad
  • 99bdd736
  • 23f40db0
  • 74e59edd
  • 00db1d69

The main changes start at 2eb09f4f, which rewrites the circuit reactor main loop to use select_biased! to poll multiple futures simultaneously.

The new run_once(), like the old, first waits for an initial CtrlMsg::Create. Then, it uses a select_biased! to poll the chan_sender sink and shutdown channel for readiness.

When the channel sink is ready, we poll the control and input channels like before, as well as the new ready_streams Stream (ready_streams is a futures::Stream that replaces the previous send_outbound() function).

Most of the implementation remains unchanged, except the handle_input, handle_cell and handle_control functions no longer send anything on the chan_sender channel. Instead, they may do some (synchronous) processing, and send instructions for the remaining work that needs to be done (for example, for writing the cell to the chan_sender channel). These instructions are handled at the end of run_once(), and are encoded in the RunOnceCmdInner enum.

What this change does not do:

• the control channel still bypasses congestion control. We could fix this by making the various reactor functions send the RunOnceCmdInner commands to run_once() via a channel (instead of returning them). This would enable the reactor to stop reading the commands (except for handle Sendme, which would be handled separately) if it's blocked on congestion control.