Refactor data model to use config_id instead of datastore in xxx_object tables

These tables used to reference the datastore the invalid/missing object
was found in, but not keeping the config entry, i.e. the checking session
during wich the invalid/missing object was found, which can be an issue
when more than one checking session is executed on a given datastore.

This replaces the `datastore` field of tables `corrupt_object`,
`missing_object` and `missing_object_reference` tables by `config_id`.

Adapt all the code accordingly.

Note that it changes a bit the cli usage: the kafka checker now needs
a config entry, thus a kafka checking session can ony target a given
object type (i.e. one kafka topic),

The migration script will fill the config_id column for corrupt_object
using the check_config entry that matches the oject_type (of
corrupt_object) and datastore. For missing_object and
missing_object_reference, it will use this later table to idenify the
check_config entry corresponding object type for the reference_id and
datastore, since it is a checking session on this object type that will
generate a missing object entry (which is generaaly not of the same
type). For the missing_object table, the config_id will use the one
extracted from the missing_object_reference (joining on the missing_id
column).

Note that the migration script will fail if there are rows in one
of these tables for which there exists more than one possible
config_entry (i.e. with the same object_type and datastore).