Feature/client refactoring

[jstuczyn]

• Closes Wherever unbounded mpsc channel is used, prefer unbounded_send() over send().await #90
• Experimentally closes Add a Send method in nym-client #81 (needs some testing)
• Closes Reuse TCP socket connection between client and mixnodes #20

[futurechimp]

This one works locally but has a problem in CI (only beta and nightly). We've investigated for quite a while, and can only find linkerd/linkerd2#777 as something similar (Rustaceans seeing flakey TCP connection timeout tests sporadically in CI). Our test failure is rock-solid, but we can't repro it outside of the Travis environment, so we're (reluctantly) going to mark beta as ok to fail.