lnrpc: deprecate QueryRoutes with more than one route

Now that QueryRoutes gained the ability to route from any source node
and takes in edge and node black lists, all pieces are in place to have
users implemented their own k-shortests path algorithm. Or any other algorithm
they might wish to use and currently can't.

This commit marks the num_routes field as deprecated as a preparation
for removing k-shortest for lnd.

lnrpc/rpc.proto

@@ -1467,8 +1467,11 @@ message QueryRoutesRequest {
     /// The amount to send expressed in satoshis
     int64 amt = 2;
 
-    /// The max number of routes to return.
-    int32 num_routes = 3;
+    /**
+    Deprecated. The max number of routes to return. In the future, QueryRoutes
+    will only return a single route.
+    */
+    int32 num_routes = 3 [deprecated = true]; 
 
     /// An optional CLTV delta from the current height that should be used for the timelock of the final hop
     int32 final_cltv_delta = 4;

lnrpc/rpc.swagger.json

@@ -564,7 +564,7 @@
           },
           {
             "name": "num_routes",
-            "description": "/ The max number of routes to return.",
+            "description": "*\nDeprecated. The max number of routes to return. In the future, QueryRoutes\nwill only return a single route.",
             "in": "query",
             "required": false,
             "type": "integer",