Clear transaction tasks on retry of with_transaction decorator

Closes #13008
(grafted from 6ba5facee41af77e391409382d9aef7a6b4b576a)

--HG--
branch : 6.0

trytond/trytond/protocols/wrappers.py

@@ -212,6 +212,7 @@ def with_transaction(readonly=None, user=0, context=None):
                     except backend.DatabaseOperationalError:
                         if count and not readonly_:
                             transaction.rollback()
+                            transaction.tasks.clear()
                             continue
                         logger.error('%s', request, exc_info=True)
                         raise