It is not possible to connect to an IPv4 address from an IPv6 address or
vice-versa so don't waste time trying. If no remote addresses match then a
"missing connect error" will now occur.
Closes#1903
BREAKING CHANGE: The `Resolve` trait is gone. All custom resolves should
implement `tower::Service` instead.
The error type of `HttpConnector` has been changed away from
`std::io::Error`.
This takes the same strategy as golang, where the timeout value is
divided equally between the candidate socket addresses.
If happy eyeballs is enabled, the division takes place "below" the
IPv4/IPv6 partitioning.
The `HttpConnector` and `AddrListener` types which make use of
`tokio::tcp` have been made their own optional feature. This allows
using them without requiring the *full* tokio runtime.
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
