This introduces a `Resolve` trait to describe asynchronous DNS
resolution. The `HttpConnector` can be configured with a resolver,
allowing a user to still use all the functionality of the
`HttpConnector`, while customizing the DNS resolution.
To prevent a breaking change, the `HttpConnector` has its `Resolve`
generic set by default to `GaiResolver`. This is same as the existing
resolver, which uses `getaddrinfo` inside a thread pool.
Closes#1517
- Adds `client::connect::Connected::extra()`, which allows connectors to
specify arbitrary custom information about a connected transport.
If a connector provides this extra value, it will be set in the
`Response` extensions.
Closes#1402
