Add gunicorn threads

[jsangmeister]

With this change, it should be possible to continue to receive action worker updates even when the datastore is currently busy.

• The writer still runs in a single worker (=process), so the singleton lock principle is kept and locked fields continue to work as intended.
• This worker now has multiple threads, so even if one is occupied with a long query, the other threads can continue to write action worker updates without the lock or reserve ids.

A possible problem is that multiple "normal" write requests come in after some long-running request, blocking the remaining x threads (4 by default). Possible solutions include increasing the number of threads (but how high?), spawning a seperate container analogous to the backend-manage container which only handles writes without events (will be more work than this, not sure if it's worth it) or maybe adding a timeout to the lock acquirement of the normal write route - if the lock is not available after 1 second or something, simply terminate the request (might need some refinement, as the backend automatically re-tries failed requests, so they will appear again quickly in the datastore; also, it makes the user experience worse, as currently, all requests simply wait until the long-running one is done, while after this solution they will come back as an error to the user, who will have to try again manually).

I think for now this simple solution should suffice, maybe with an increased number of threads, depending on how many threads will be typically used in production.