Set timeout to 0 when receiving PostgreSQL notifications

There is no need to wait 2 seconds for more notifications as we call
get_notifications only when the selector is triggered.

Closes #15102
(grafted from 02567102eb63f69faedf61ae83c51a5075b57588)

--HG--
branch : 8.0

trytond/trytond/backend/postgresql/database.py

@@ -480,7 +480,7 @@ class Database(DatabaseInterface):
         cursor.execute('SELECT pg_notify(%s, %s)', (channel, payload))
 
     def get_notifications(self, connection):
-        return list(connection.notifies(timeout=2))
+        return list(connection.notifies(timeout=0))
 
     def lock(self, connection, table):
         cursor = connection.cursor()