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
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