accounts: Enable checking if an invoice has been settled or not through the `accounts` cli

[ViktorTigerstrom]

Currently, the accounts subsystem (and its db) itselft has no way to determine whether an invoice is settled. The subsystem relies on the LND client and the last settled invoice index to identify which invoices to check for settlement.

That therefore requires that the super user queries LND to determine if a specific invoice created for an account has settled or not.
There would be value in simplifying the user experience, where a superuser of the accounts subsystem wouldn't need to query LND to check whether a specific invoice has been settled.