Admit a peer network #75

The other half of the handshake. Read a candidate network's two documents, check the domain matches, verify the signature, check the revision has increased, and record the decision locally. Admission is directional and local — A admitting B says nothing about B admitting A, and neither network writes to the other's records. A peer publishing a new document must not be able to widen or restore its own admission. A notification about a network or key change is only a hint to re-read and re-verify the documents from their fixed location.

Doc: Handshake, steps "Validate the candidate" and "Admit the peer"; Notify (network or key change).

Parent: #48