split Client into (Client, Connection) (#107)

The Connection type is a `Future` that drives all of the IO of the
client connection.

The Client type is separate, and is used to send requests into the
connection.

src/proto/streams/mod.rs

@@ -10,6 +10,7 @@ mod stream;
 mod streams;
 
 pub(crate) use self::prioritize::Prioritized;
+pub(crate) use self::store::Key;
 pub(crate) use self::streams::{StreamRef, Streams};
 
 use self::buffer::Buffer;