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/mod.rs

@@ -8,7 +8,7 @@ mod streams;
 pub(crate) use self::connection::Connection;
 pub(crate) use self::error::Error;
 pub(crate) use self::peer::Peer;
-pub(crate) use self::streams::{StreamRef, Streams};
+pub(crate) use self::streams::{Key as StreamKey, StreamRef, Streams};
 
 use codec::Codec;