Add a weakly-counted context wrapper

[peterjoel]

This need arose when we had some code like this:

lazy_static! {
    static ref ZMQ_CTX: zmq::Context = zmq::Context::new();
}

Which is so we can reuse the context conveniently without have to pass it around. The problem was that some final messages would not be sent, because the context's finalizer would never run.

We could work around this with:

lazy_static! {
    static ref ZMQ_CTX: Mutex<Weak<Context>> = Mutex::new(Weak::new());
}

fn zmq_ctx() -> Arc<Context> {
    let mut ctx = ZMQ_CTX.lock().unwrap();
    if ctx.strong_count() == 0 {
        let new_ctx = Arc::new(zmq::Context::new());
        *ctx = Arc::downgrade(&new_ctx);
    }
    ctx.upgrade().unwrap()
}

But this is adding an additional layer of Arcs, which shouldn't be necessary. With the PR change, we can do something like this instead:

lazy_static! {
    static ref ZMQ_CTX: Mutex<WeakContext> = Mutex::new(WeakContext::new());
}

fn zmq_ctx() -> Context {
    let mut ctx = ZMQ_CTX.lock().unwrap();
    let new_ctx = Context::new_from_weak(&ctx);
    *ctx = new_ctx.to_weak();
    new_ctx
}

[peterjoel]

Alternatively, we could just expose RawContext.