Handle new mandates from existing donors better

[hanneskod]

• If the new mandate concerns a different account there could be a choice to revoke the old mandate and queue the new one for addition.
• If the new mandate concerns the same account but has altered fields (amount, address and so forth), an appropriate update-cascade could start.
• If a mandate is not-processed if could be broken out in some whay (think XML-import with multiple mandates in the same faile of which only one should not be processed.