tor-circmgr: Remove Arc around ClientCirc

Copying the rationale from some new commentary:

#[derive(Clone, Debug)]
/// A circuit that we have constructed over the Tor network.
///
/// This struct is the interface used by the rest of the code, It is fairly
/// cheaply cloneable.  None of the public methods need mutable access, since
/// they all actually communicate with the Reactor which contains the primary
/// mutable state, and does the actual work.
//
// Effectively, this struct contains two Arcs: one for `hops` and one for
// `control` (which surely has soemthing Arc-like in it).  We cannot unify
// these by putting a single Arc around the whole struct, and passing
// an Arc strong reference to the `Reactor`, because then `control` would
// not be dropped when the last user of the circuit goes away.  We could
// make the reactor have a weak reference but weak references are more
// expensive to dereference.
//
// Because of the above, cloning this struct is always going to involve
// two atomic refcount changes/checks.  Wrapping it in another Arc would
// be overkill.
pub struct ClientCirc {

This builds on !219 (merged) which I have just merged.