Make using tokio as the executor easier

[huuff]

Today I needed to use a pretty non-standard configuration for lapin, and it was very hard to set up. I had difficulties setting up tokio as the executor and also using rustls with webpki root certs, but I think it's maybe better to focus on one issue at a time.

My biggest issue switching the runtime to tokio was requiring two other dependencies in the amqp-rs/ namespace (tokio-executor-trait and tokio-reactor-trait) and also ensuring they were in-sync with lapin itself.

I think that maybe lapin could offer a new tokio feature, that depends on these two (just like default-runtime does) and maybe also provide a ConnectionProperties constructor that automatically sets them up? Or maybe just using them in the Default implementation if the feature is enabled?

I may try this approach in a fork to simplify my downstream configuration. If it's ok by you, I could also work on a PR.

Thanks for your work on this project, by the way!

[huuff]

I've got a working PoC here https://github.com/huuff/lapin/tree/tokio-runtime

The entrypoint is:

#[cfg(not(feature = "tokio-runtime"))]
impl Default for ConnectionProperties {
    fn default() -> Self {
        Self {
            locale: "en_US".into(),
            client_properties: FieldTable::default(),
            executor: None,
            reactor: None,
            recovery_config: None,
        }
    }
}

#[cfg(feature = "tokio-runtime")]
impl Default for ConnectionProperties {
    fn default() -> Self {
        Self {
            locale: "en_US".into(),
            client_properties: FieldTable::default(),
            executor: Some(Arc::new(tokio_executor_trait::Tokio::current())),
            reactor: Some(Arc::new(tokio_reactor_trait::Tokio)),
            recovery_config: None,
        }
    }
}

When the tokio-runtume is enabled, the appropriate executor and reactor are passed-in by default, without requiring any set-up from the user. If this looks fine, I could roll a PR, but I'm also happy to make any necessary changes