core: when running as worker, we only use a single thread (per worker)

[eckter]

The logs mention 16(?) threads when starting up, but we only use one.

To test this, the simplest way to test it is to add a Thread.sleep when processing messages (with lots of messages in the queue). It's easy to tell that the sleep freezes the whole thing, it doesn't process other messages in the meantime.

Ideally, we should enable proper multithreading with ways to configure it. It would be much faster when running the app locally, while still having a single infra loaded in memory.

Note: we may run into issues where several messages initiate an infra loading at the same time. InfraManager is supposed to be robust to this case though.