♻️ REFACTOR: Remove QueryManager (#5101)

The `AbstractQueryManager` class and its implementations, 
`DjangoQueryManager` and `SqlaQueryManager`, is an odd mix of a few
methods, with no real conceptual purpose; essentially a catch-all for
things that didn't have another home.

In this PR, the classes are removed and their methods are reassigned as
appropriate:

 * `get_creation_statistics` has been moved to the `BackendQueryBuilder`
 * `get_duplicate_uuids` and `apply_new_uuid_mapping` have been moved to
   `aiida/backends/general/migrations/duplicate_uuids.py`. They were
   defined both in `aiida/backends/general/migrations/utils.py` as well
   as in `aiida/manage/database/integrity/duplicate_uuid.py`. Note that
   the former had been adapted after the repository redesign, but the
   latter was omitted by mistake. The latter is now correctly updated to
   contain compatibility for the new repository. Since the methods
   directly use SQL, they cannot be easily abstracted and moved to the
   `BackendQueryBuilder`.
 * `get_bands_and_parents_structure` has been moved to the ORM module
   in `aiida/orm/nodes/data/array/bands.py`.

aiida/backends/djsite/db/migrations/0014_add_node_uuid_unique_constraint.py

@@ -27,7 +27,7 @@ def verify_node_uuid_uniqueness(_, __):
 
     :raises: IntegrityError if database contains nodes with duplicate UUIDS.
     """
-    from aiida.backends.general.migrations.utils import verify_uuid_uniqueness
+    from aiida.backends.general.migrations.duplicate_uuids import verify_uuid_uniqueness
     verify_uuid_uniqueness(table='db_dbnode')
 
 

aiida/backends/djsite/db/migrations/0018_django_1_11.py

@@ -33,7 +33,7 @@ def _verify_uuid_uniqueness(apps, schema_editor):
     :raises: IntegrityError if database contains rows with duplicate UUIDS.
     """
     # pylint: disable=unused-argument
-    from aiida.manage.database.integrity.duplicate_uuid import verify_uuid_uniqueness
+    from aiida.backends.general.migrations.duplicate_uuids import verify_uuid_uniqueness
 
     for table in tables:
         verify_uuid_uniqueness(table=table)