This corrects the documentation for the `amt_paid` and `amt_paid_msat`
fields on the `Invoice` message to indicate that the fields will be set
if the state of the invoice is either accepted or settled, not only
settled. This reflects the actual behavior of lnd, as demonstrated in
the below `lncli` output:
```
"amt_paid": "10000",
"amt_paid_sat": "10",
"amt_paid_msat": "10000",
"state": "ACCEPTED",
```
Add additional comment to `lnclic addinvoice`'s `--private` flag warning
about including a large number of private channel hints when using zero
ammounts.
Also added a comment to the `private` field in the `Invoice` proto
message.
In this commit, we add a new `LookupInvoiceV2` method attached to the
invoice RPC sub-server. Compared to the existing version, this version
allows an invoice to be looked up by an invoice ref. This enables users
to query an AMP invoice based on a specific _set ID_, so they can get
the information related to the set of cumulative settles to that
invoices.
We also test that legacy keysend payments are promoted to AMP payments
on the receiver-sdie by asserting basic properties of the fields
returned via the rpc.
