[rel-db] DB-connection handling for actions

[peb-adr]

As writing to the DB will no longer happen in the datastore-service, the backend must initiate it's own connections for writing to DB.

We discussed that the connection context and transaction should be opened in the action handler before the action specific code is called.
Therefore the get_connection function of the db_connection_handling module should return a context manager which can then be used within the action handler.

[luisa-beerboom]

@hjanott said this is done