Allow client IO to be !Send

[seanmonstar]

Just like server::conn::http1::Connection can work with an IO type that isn't Send, because it has a with_upgrades() variant, we should make it so the client http1::Connection type doesn't require Send. This would be a breaking change, and so should happen before tagging 1.0.

I believe doing this is probably somewhat involved, let me lay out what I think needs to be done. If any step is unclear, or another way seems better, please comment :)

• Remove Send from the T: Read + Write bounds for conn::http1::Connection
• Provide a UpgradeableConnection in similar style to what is in server.
• Update the single_threaded.rs example to use a !Send IO type for the client, to prove it works.