Pub/Sub Event Store

[DavidBadura]

Postgres has a Pub/Sub function that we can use for the ProcessManager and Watcher. So that we don't have to poll all the time.

TBD: I think a new interface would be necessary here, for example. SubscribableStore, with a function called subscribe().

But the challenge is how we solve this with the scheme. Doctrine schema does not have the ability to define triggers etc. This also means we won't get it in Doctrine Migration either.

We also have to see whether the places where we could use the Pub/Sub feature need extra logic.