Add federation

[jshufro]

Make rescue-api horizontally scalable by introducing a peering mechanism that distinct instances can use to communicate credential events.

First, add a new cli arg that takes a vector of peer URIs
Add a new cli arg that takes a single URI on which to handle requests from peers
Next, set up a grpc bidirectional streaming handler on that URI with a message with the same arity as a credential_event

When a new row would be inserted into credential_event, replicate it to peers by sending it to each one in the vector, as well as to any connected on the bidirectional endpoint.

When a message is received on the bidirectional endpoint, store it to credential_event, doing nothing on conflict.