routerrpc: default timeout_seconds to 60 in SendPaymentV2

If timeout_seconds is not set or is 0, the default value
of 60 seconds will be used.

Signed-off-by: Nishant Bansal <nishant.bansal.282003@gmail.com>

lnrpc/routerrpc/router.proto

@@ -227,10 +227,11 @@ message SendPaymentRequest {
     string payment_request = 5;
 
     /*
-    An upper limit on the amount of time we should spend when attempting to
-    fulfill the payment. This is expressed in seconds. If we cannot make a
-    successful payment within this time frame, an error will be returned.
-    This field must be non-zero.
+    An optional limit, expressed in seconds, on the time to wait before
+    attempting the first HTLC. Once HTLCs are in flight, the payment will
+    not be aborted until the HTLCs are either settled or failed. If the field
+    is not set or is explicitly set to zero, the default value of 60 seconds
+    will be applied.
     */
     int32 timeout_seconds = 6;