Define a "mostly bounded queue" type

Several places in tor-proto, we have a pattern where we combine a bounded MPSC queue with an unbounded VecDeque: Mostly we use the bounded queue, but occasionally, when we have a must-send message and the bounded queue is full, we use the unbounded VecDeque.

This could be reimplemented using a "mostly bounded" queue: such a queue would be internally unbounded, but have a "soft bound". It would provide a Sink implementation that would respect the soft bound, and a send_unbounded implementation that would send unconditionally.

This would ideally obviate the need for these three queues.

• CircHop::outbound.
• circuit::Reactor::outbound.
• channel::reactor::SpecialOutgoing

@Diziet says they have ideas about how to build one of these.