Remove RuntimeClient trait, simplify lifetimes in Runtime.

[davidbarsky]

Description of changes:

• This change removes the RuntimeClient trait. This allows the runtime to own the RuntimeClient directly instead of through a trait object.
• This reduces the number of complex lifetime bounds in the client and runtime.
• This also allows us to more easily support systems like Tower in the future.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[sapessi]

I was using a trait for the runtime client to be able to mock it in function tests. Do you think that's not necessary?

[davidbarsky]

I don't think it's strictly necessary as per: http://smallcultfollowing.com/babysteps/blog/2018/09/24/office-hours-1-cyclic-services. Additionally, due to limitations around existential types in traits, we can't make traits have async methods.

However, I'm curious as to how you're mocking the client in tests.

[sapessi]

Ship it!