Parallelize update script

[m-rtijn]

If we start a separate thread per webshop, we can still keep the amount of requests/hour low per webshop, but still in a faster way check all products in the database

[m-rtijn]

The parallelization script is currently broken for MariaDB backends. This is due to mistakes in the SQLAlchemy backend handling.

Another possible improvement to the update script would be not to wait when no update was executed, or possibly just only querying not recently updated ProductOffers in the database.