Add way of creating a rpc::Client without performing a health check

[romac]

In the ibc-rs crate, we need to create a rpc::Client in a non-async context in order to store it for later use within a struct, and having rpc::Client::new perform an async health check on creation makes this impossible without spawning an executor, and executing the task returned by new.

I would suggest renaming the existing method to new_healthy and adding a plain new method which does not perform a health check.

[liamsi]

resolved in #169