We now use the jsonpb marshaler to convert the RPC responses to
JSON in lncli and REST. The jsonpb has a setting to use the
original name as defined in the proto file and the explicit
json_name definition is not necessary any more.
The jsonpb setting is called OrigName and needs to be true.
This fixes a bug where the RHashStr field on the PaymentHash request for
SubscribeSingleInvoice was being ignored. This replaces the PaymentHash
message for the SubscribeSingleInvoice request with a new message called
SubscribeSingleInvoiceRequest that contains only the RHash field.
