Fix race condition with logical slot advance and copy (#3098)

The `SlotsAdvanceThread` is asynchronously calling
pg_replication_slot_advance() and providing feedback about logical
replication slots that must be reinitialized by copying from the
primary. That is, the parent thread will learn about slots to be copied
only when scheduling the next pg_replication_slot_advance() call.
As a result it was possible situation when logical slot was copied with
PostgreSQL restart more than once.

To improve it we implement following measures:
1. do not schedule slot sync if it is in the list to be copied
2. remove to be copued slots from the `self._scheduled` structure
3. clean state of `SlotsAdvanceThread` when slot files are copied.

tests/test_slots.py

@@ -274,6 +274,10 @@ class TestSlotsHandler(BaseTestPostgresql):
                 self.s.schedule_advance_slots({'foo': {'bar': 100}})
                 self.s._advance.sync_slots()
                 self.assertEqual(self.s._advance._copy_slots, ["bar"])
+                # we don't want to make attempts to advance slots that are to be copied
+                self.s.schedule_advance_slots({'foo': {'bar': 101}})
+                self.assertEqual(self.s._advance._scheduled, {})
+                self.s._advance.clean()
 
         with patch.object(SlotsAdvanceThread, 'sync_slots', Mock(side_effect=Exception)):
             self.s._advance._condition.wait = Mock()