Create multiple contacts

Creates multiple contacts and updates existing ones.
This route uses WebSocket in order to notify connected clients of the creation progress
The route process is the following:

1. The webSocketServer (wss) sends one contact_bulk_create_starting
2. wss sends as many contact_bulk_create_running as there is contacts to create/update
3. wss sends one contact_bulk_create_ending
   The API returns between the sending of _starting and _ending
   WebSocket payload format:

• _starting: {
  listId: Id of the list in which contacts will be added,
  uuid: Unique transaction id
  }
• _running: {
  uuid: Unique transaction id,
  i: Index of the contact currently processed,
  total: Total number of contact to process
  }
• _ending: {
  uuid: Unique transaction id,
  created: Number of contact created
  updated: Number of contact updated
  }