Follow-ups to PR #658

[tnull]

In #658 we switched to use BDK wallet events to update the payment store and introduced a new 'pending payement store'. We merged the PR, though here are some follow-ups I want to address before the next release:

• io/mod.rs: Move persistence consts up to other payment consts
• DRY up read_ method
• Don't expose PendingPaymentDetails in public API
• Move PendingPaymentStore to payment_store.rs
• Make conflicting_txids a Vec rather than Option<Vec<>>
• Not convinced we need to track entire PaymentDetails in the pending store, plus semantically it doens't make sense to track conflicts for BOLT11 payments for example. Maybe there is another way?
• Do we really need to track pending payments in both stores side-by-side?