Composition instead of inheritance for PostgresJobStore

[ewjoachim]

We're dealing with fine software design issues, it's really no big deal if we don't go this way.

I think we could make a last (haha, last...) modification on how our job store handles being plugged with a real Postgres database versus being in memory for tests and such . Today, it's done via inheritance, with a BaseJobStore, a PostgresJobStore and an ImMemoryJobStore. I've finally realized that it would be much cleaner in terms of separation of concerns if the whole idea of store was uncorrelated from the idea of DB connection. We could have a JobStore, and a PostgresConnector and an InMemoryConnector, and the JobStore would receive an ...Connector instance at creation time (same way as the App currently receives a Job Store instance).

That would mean the App would be given a connector instead of a JobStore. If we change the name, that would be backwards incompatible, but we could still keep the name PostgresJobStore as an alias of PostgresConnector for at least a few releases.