[WIP] Add worker name metadata

[francesconazzaro]

This PR adds the information on where (on which worker) a request is running.
This implementation is an alternative to this one: #96.

Pros:

• the information of the worker is inside the system_request table and is easily accessible by this line

Cons:

• The broker has to write the worker name info when the request is submitted to the worker. This doubles the number of database writes from the broker.

This PR has to be merged with ecmwf-projects/cads-worker#33.