Prevent StreamingPayment uninstallation if pending streams remain

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The main feature here is to prevent uninstallation of the extension in the event that there are some streaming payments unclaimed. It also:

• Reworks some calculations to prevent over/underflows. This is somewhat opinionated in that the intention is to 'cap' the calculations
• Allows editing of the interval, which is a requirement. While in an ideal world, editing the interval shouldn't be necessary as you could edit the amount, rounding errors meant that this should be editable as well. This is possible as we have moved to single-stream payments.