channeldb: store AMP invoice htlc sets in new prefix key near invoice
channeldb: store AMP invoice htlc sets in new prefix key near invoice

In this commit, we modify the HTLC storage for AMP invoice only to be
stored within a new key prefix next to the main invoice data. We do this
as otherwise each time an AMP invoice is settled (enabled by the
following commits), we need to continually encode+decode the _entire_
set of invoices.

Instead, we store the AMP invoices within the main invoice bucket, using
the invoice number as a key prefix, with the final key being:
`invoiceNum || setID`

aakselrod/lnd,aakselrod/lnd