Create async Send alternative

[nappa85]

As asked in #55, there is need for a Send version of the async builders.
This PR introduces it aside of standard builders, this way old code won't be changed and it's up to the user to choose the right variant.
Obviously comments are welcome

[someguynamedjosh]

I have merged this but will avoid publishing it until there is confirmation that this is safe to do.

[someguynamedjosh]

@petrosagg as the original author of the async builder code, can you confirm or deny if allowing Send on the builder is safe?

[petrosagg]

This looks perfectly safe to me! It's a bummer the type system forces us to have two separate methods but I don't think this can be fixed without removing the Box<dyn Future<..>>. It's the same reason the async_trait crate has an explicit option to control Send, because internally it also uses Pin<Box<dyn Future<..>>> types https://docs.rs/async-trait/latest/async_trait/#non-threadsafe-futures

One day..