Do not drop active replication slots.

Master tried to delete all slots that did not correspond to the
replica registered in Patroni. That produced an error for the slots
that were active, potentially preventing drop and creation of other
slots.

Reported by Murat Kabilov.

patroni/postgresql.py

@@ -884,7 +884,7 @@ $$""".format(name, ' '.join(options)), name, password, password)
                 for slot in set(self._replication_slots) - set(slots):
                     self.query("""SELECT pg_drop_replication_slot(%s)
                                    WHERE EXISTS(SELECT 1 FROM pg_replication_slots
-                                   WHERE slot_name = %s)""", slot, slot)
+                                   WHERE slot_name = %s AND NOT active)""", slot, slot)
 
                 # create new slots
                 for slot in set(slots) - set(self._replication_slots):