[WIP] Create events table for live logs and worker name

[francesconazzaro]

This PR adds events table to the broker database to store events related to a request. The table has 5 columns:

• event_id: id of the event
• event_type: type of the event. For example worker_name, user_visible_log, log etc...
• request_uid: the request uid of the request, this is a foreign key to system_request.request_uid.
• message: the content of the event.
• timestamp: creation timestamp of the event.

Pros and cons for the worker name info (see this PR).
Pros:

• the broker code workflow remains unchanged

Cons:

• the information of the worker is outside the system_request table and is harder to access by this line

This PR has to be merged with ecmwf-projects/cads-worker#32.