Sync and Async/Await

[dbcfd]

Async/Await code should use an async concurrency primitive (e.g. tokio::sync::RwLock).

Non async primitives will produce errors like

`std::sync::RwLockReadGuard<'_, ratsio::stan_client::ClientInfo>` cannot be sent between threads safely

Additionally, since they do not wake the executor, they can cause threads in the executor to enter infinite wait states.

[giacomocariello]

dbcfd seems to fix the issue successfully. Could @mnetship please accept the PR and possibly publish a new release on crates.io?

[ebkalderon]

I'm glad that there already exists a pull request which switches from blocking std::sync::RwLock to non-blocking tokio::sync::Mutex. I was about to open an issue and PR to fix this myself. Nice to see someone else got to it first! It's a big blocker for me using this library. 😄