Support running multiple (name)servers

[ncruces]

Running multiple instances of keyless-server should be possible as long as:

• important bits of config.json are kept in sync:
  • domain, nameserver, cname, api.handler
• master and legacy keys are made available to all servers:
  • master key doesn't need to be the same for all servers, but
  • all keys must be available to all servers.

The remaining difficulty is to help servers in a "cluster" solve ACME challenges cooperatively, to ensure certificate issuance and renewal work.