Implement asynchronous responses

[ebkalderon]

Added

• Create ResponseFuture and PreResolving types.
• Add wait_for_response() methods to certain futures, which drives them to completion using a tokio::Runtime and returns a Result, restoring the original synchronous behavior.

Changed

• Make Client, Connector, and Resolver fully asynchronous.
• Update unit tests and get.rs example to use wait_for_response().
• Add Send + Sync bounds on DnsResolver trait.
• Change DnsResolver to return a Resolving future.
• Removed some unnecessary clones by taking ownership instead of borrowing.
• Add #[must_use] attribute to all futures.
• Flesh out doc comments.
• Run cargo fmt on the codebase.

Removed

• Delete unused AsyncResolver and AsyncResolverFuture types.
• Remove dependency on trust-dns-proto crate.

Closes #10.

[kpcyrd]

@ebkalderon very nice work!

I've changed one of the projects using chrootable-https to your branch and everything worked out of the box after adding .wait_for_response() a few times.

I've pushed a branch called async that contains one additional commit to remove the pre_resolve workaround that's not needed anymore due to your changes. Could you review if that change works for you?

[ebkalderon]

Looks good to me! I've pulled your change to my branch to get it tested by Travis, just so it's ready to merge.

One thing that's missing in this PR but I think might eventually make for a nice addition is the ability to specify a custom executor for the connector, instead of relying on the 4 default threads created by the HttpsConnector, but this relies on rustls/hyper-rustls#55 being resolved first. If hyper-rustls gets updated, I may make a follow-up PR, if you're interested.

[kpcyrd]

@ebkalderon thanks for all your work!

[ebkalderon]

@kpcyrd No problem! Thanks for the very useful library.