When duplicatePaymentSequenceKey is missing from a duplicate payment
sub-bucket, the code returned the outer function's err variable which
is nil at that point. This caused corrupted duplicate entries to be
silently treated as "not found" instead of failing loudly.
Return a new dedicated ErrNoDuplicateSequenceNumber error so malformed
data is detected immediately.
We now return an error when blinded and non blinded attempts are
combined. This was theoretically possible to register a legacy
attempt in combination with a blinded payment. This would have
been prevented by other checks in the code because legacy payments
are not split into shards.
We introduce a new package paymentsDB and start by moving the
payment specifc errors from the channeldb package to the
paymentsDB package. We also fix linter issues which showed up
due to changing this code part.