Consider simplifying/refactoring PendingPaymentStore #950

As part of #770 we considered having the PendingPaymentStore not track full PaymentDetails. While we had already implemented that simplification as part of #937 it turned out in #948 that for the time being we might need them around. However, after all of the currently open PRs 'in that area' (#888, #948, #937) have landed we should revisit whether we can simplify / refactor the pending payment store. In particular, we should do so before shipping v0.8 as we're still free to break API and persistence before that.