Finish refactoring of the Status class (#3103)

The `Status` class was introduced in #2853, but we kept old properties in the `Cluster` object in order to have fewer changes in the rest of the code.

This PR is finishing the refactoring.
The following adjustments were made:
- Introduced `Status.is_empty()` method, which is used in the `Cluster.is_empty()` instead of checking actual values to simplify introduction of further fields to the Status object.
- Removed `Cluster.last_lsn` property
- Changed `Cluster.slots` property to always return dict and perform sanity checks on values.

Besides that, this PR addressing a couple of problems:
- the `AbstractDCS.get_cluster()` method some properties without holding a lock on `_cluster_thread_lock`.
- `Cluster.__permanent_slots` property was setting 'lsn' from all cluster members, while it should be doing that only for members with `replicatefrom` tag.

tests/test_slots.py

@@ -213,7 +213,7 @@ class TestSlotsHandler(BaseTestPostgresql):
     @patch.object(Postgresql, 'is_primary', Mock(return_value=False))
     def test__ensure_logical_slots_replica(self):
         self.p.set_role('replica')
-        self.cluster.slots['ls'] = 12346
+        self.cluster.status.slots['ls'] = 12346
         with patch.object(SlotsHandler, 'check_logical_slots_readiness', Mock(return_value=False)):
             self.assertEqual(self.s.sync_replication_slots(self.cluster, self.tags), [])
         with patch.object(SlotsHandler, '_query', Mock(return_value=[('ls', 'logical', 499, 'b', 'a', 5, 100, 500)])), \
@@ -222,10 +222,10 @@ class TestSlotsHandler(BaseTestPostgresql):
                 patch.object(psycopg.OperationalError, 'diag') as mock_diag:
             type(mock_diag).sqlstate = PropertyMock(return_value='58P01')
             self.assertEqual(self.s.sync_replication_slots(self.cluster, self.tags), ['ls'])
-        self.cluster.slots['ls'] = 'a'
+        self.cluster.status.slots['ls'] = 'a'
         self.assertEqual(self.s.sync_replication_slots(self.cluster, self.tags), [])
         self.cluster.config.data['slots']['ls']['database'] = 'b'
-        self.cluster.slots['ls'] = '500'
+        self.cluster.status.slots['ls'] = '500'
         with patch.object(MockCursor, 'rowcount', PropertyMock(return_value=1), create=True):
             self.assertEqual(self.s.sync_replication_slots(self.cluster, self.tags), ['ls'])