rust-chg: reimplement attach_io operation as async function In short, MessageLoop<Connection> was redesigned as Protocol<Connection>, and the protocol methods no longer consume self. API changes are briefly documented in the following page: https://docs.rs/tokio-hglib/0.3.0/tokio_hglib/struct.Protocol.html Differential Revision: https://phab.mercurial-scm.org/D8442