Implement the interaction with the prekey server about libotr-ng

[claucece]

As it would probably be defined here: https://github.com/otrv4/otrv4-prekey-server

[claucece]

This includes (maybe):

• Structure of prekey server.
• Validation of each prekey message returned by server.
• DAKEZ with server: when does it start?
• Query/or similar to server.
• Incorporate a sending data message on non interactive msg from client.
• Questions:
  • can the enc msg on non interactive auth append tlvs? No
  • can the enc msg on non interactive auth have flags? No
  • will the receive of enc msg advertise a new ratchet? Yes

[claucece]

For the moment, please, don't look at this issue. It should be moved once we move the work to the prekey server spec is done.

[claucece]

This is duplicate of #98 . I'm closing this.