- Adds `Connected::negotiated_h2()` method to signal the connection must
use HTTP2. `Connect` implementations should set this if using ALPN.
If a connection to a host is detected to have been upgraded via ALPN,
any other oustanding connect futures will be canceled, and the waiting
requests will make use of the single HTTP2 connection.
The `http2_only` builder configuration still works the same, not
requiring ALPN at all, and always using only a single connection.
976a77a673