Lunchtime stab at adding temporary halts on payments

[the-frey]

Had a spare half hour. Initially went the route of removing payments but that approach would result in dodgy behaviour (I think!) when the following scenario occurs:

• Halt a payment because of non delivery before payment date
• Resolve dispute before payment date
• Wish to reinstate payment before payment date

Therefore the non-destructive approach with filtering seemed to make more sense.

Hence putting it in a separate piece of state.

TODO:

• Needs some unit tests
• Versions are out-of-date so need to bump & fix

Relates to #5

[vernonjohnson]

I like this approach. Only question is whether or not the halting should be a bit more granular. You could halt individual payments, rather than all payments to a recipient. And if you wanted to halt all payments to a specific recipient, you can pass in multiple payment ids to do so, which gives you feature parity with this draft.