lnrpc+routerrpc: disable custom tlv records via rpc

[cfromknecht]

In order to prevent future unforeseen issues, we are temporarily
disabling the ability to send custom tlv records to the receiver of a
payment. Currently the receiver does not process or expose these
additional fields via rpc or internally, so they are being disabled
until the end-to-end flow is finished and fully validated.

[joostjager]

LGTM

Only thing is what we discussed before, we are still writing a zero here

lnd/channeldb/payments.go

Line 537 in d6bbb11

if err := WriteElements(w, numRecords); err != nil {

But I don't think we easily do anything about that.

[Roasbeef]

LGTM 🎸

[Roasbeef]

Only thing is what we discussed before, we are still writing a zero here

I don't think that's an issue, as we can still project any tlv values written in the payload within the main Hop struct. Usage of the TLV format on disk lets us decouple the in-memory representation from the on-disk representation. The same goes for any of the proto structs as well.