Make client able to use non-Send executor

[seanmonstar]

This likely needs to be done before 1.0, because I suspect it involves adding a generic to a few types. But it's likely too late to fit into RC1.

The client code currently spawns a couple tasks for a connection, but does so by requiring the Executor<Box<dyn Future + Send>>, so we can spawn multiple. We can't make the internal type public, but we need to propagate the Send bounds of the IO and body types. So, we should change it to have bounds similar to the server::conn::http2, essentially an extension trait that means E: Executor<InternalFutureType<IO, B>>.

A way we can tell this all works is that we should be able to augment the single-threaded example file with a client as well.

Other relevant issues: #2341

[Ruben2424]

I will try to implement this.