Keep `context` and `client` fields for `Publish` and `Request` by reference

[caspervonb]

Avoids clones to Client and Context when publishing messages and sending requests, instead binding them to a lifetime and holding onto the required state by reference.

Supersedes https://github.com/nats-io/nats.rs/pull/866

[caspervonb]

Just broke https://github.com/nats-io/nats.rs/pull/866 into atomic commits for rebase and merge.