[server-node] Allow multiple connected clients

[LayneHaber]

Is your feature request related to a problem? Please describe.
It might be worth considering to allow multiple clients subscribing for the same public identifier, but with different URLs. One way that could be done is by adding a client-provided subscriber ID that must be unique; or by hashing the evts data to get a unique ID for every client.

This would allow multiple RestServerNodeService classes to connect to the same server-node instance with a high degree of control over the events