Server cleanups

[conradoplg]

The intention of this issue is to keep track of things we need to cleanup in the server, which we might want to split into smaller issues or not:

• Support multiple messages. The server already supports it, and Make server act as "aggregator" #265 added support for the Coordinator command line arguments, but the rest stills needs to be done, namely changing the Comms trait to support it and all code that uses it.
• Possibly create a new crate for things that help using the server (Args and Outputs structs, SessionState for the Coordinator)
• Support websockets for push notifications, so that clients don't need to keep polling receive
• Add session timeouts #375
• Don't use empty string in the message queue to identify the coordinator; use an enum instead
• Improve error handling; add an error enum and error cases for each common error #374
• Rename it to frost-server or something more descriptive (frost-box?) #373