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.proto

@@ -774,6 +774,18 @@ message GetTransactionsRequest {
 message TransactionDetails {
     // The list of transactions relevant to the wallet.
     repeated Transaction transactions = 1;
+
+    /*
+    The index of the last item in the set of returned transactions. This can be
+    used to seek further, pagination style.
+    */
+    uint64 last_index = 2;
+
+    /*
+    The index of the last item in the set of returned transactions. This can be
+    used to seek backwards, pagination style.
+    */
+    uint64 first_index = 3;
 }
 
 message FeeLimit {