When fetching an AMP invoice we sometimes filter HTLCs to selected set
IDs, however we always kept the full AMP state which is irrelevant as it
contains state for all AMP payments. This was a side effect of
UpdateInvoice needing to serialize the whole invoice when storing after
an update but it is an unwanted "feature" as users will need to filter
to relevant set when listing an AMP payment or subsribing to an update.
In this commit, we fix an inconsistent in the API related to AMP
payments. When a payment request isn't specified, we require the `--amp`
flag on the CLI to make an AMP payment. However, for payment requests,
we don't require this flag. To fix this inconsistency, we now require
the `--amp` flag to _also_ be set for payment requests.
