Prevent StreamingPayment uninstallation if pending streams remain

[area]

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.

[kronosapiens]

This is a lot of new code just to implement an uninstall check, but so it goes I guess

[kronosapiens]

Is this check necessary? Why would we allow a streaming payment to be created which was worth nothing?

[area]

We already allowed it, so I had to accommodate it. The alternative would be adding a check instead of this, so it feels like much of a wash to me. In addition, there's nothing stopping you editing a stream you create down to being worth nothing, so preventing creation feels quite arbitrary?