scaffold for server to send and receive messages

[Jakobeha]

Some details:

• The server only allows one client to connect per address, but can bind to multiple addresses.
• The server spawns a thread and ClientHandler for every client/address.
• The ZeroMQ socket type is "request/reply". There's a separate method to handle each possible initial request, but if that request starts a reply/request chain, the method can handle that using callbacks.

This PR is rebased on script-tests and has other small fixes