multi: return txns first and last indices

In this commit we introduce first and last indices for the
tranasctions returned which can be used to seek for further
transactions in a pagination style.

Signed-off-by: Abdullahi Yunus <abdoollahikbk@gmail.com>

lnrpc/lightning.swagger.json

@@ -7520,6 +7520,16 @@
             "$ref": "#/definitions/lnrpcTransaction"
           },
           "description": "The list of transactions relevant to the wallet."
+        },
+        "last_index": {
+          "type": "string",
+          "format": "uint64",
+          "description": "The index of the last item in the set of returned transactions. This can be\nused to seek further, pagination style."
+        },
+        "first_index": {
+          "type": "string",
+          "format": "uint64",
+          "description": "The index of the last item in the set of returned transactions. This can be\nused to seek backwards, pagination style."
         }
       }
     },