Add a tower-based peer implementation.
Tower provides middleware for request-response oriented protocols, while Bitcoin/Zcash just send messages which could be interpreted either as requests or responses, depending on context. To bridge this mismatch we define our own internal request/response protocol, and implement a per-peer event loop that scans incoming messages and interprets them either as requests from the remote peer to our node, or as responses to requests we made previously. This is performed by the `PeerService` task, and a corresponding `PeerClient: tower::Service` can send it requests. These tasks are themselves created by a `PeerConnector: tower::Service` which dials a remote peer and performs a handshake.
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