lnrpc: add num_zombie_chans to NetworkInfo response

lnrpc/rpc.proto

@@ -1869,6 +1869,9 @@ message NetworkInfo {
     int64 max_channel_size = 9 [json_name = "max_channel_size"];
     int64 median_channel_size_sat = 10 [json_name = "median_channel_size_sat"];
 
+    // The number of edges marked as zombies.
+    uint64 num_zombie_chans = 11 [json_name = "num_zombie_chans"];
+
     // TODO(roasbeef): fee rate info, expiry
     //  * also additional RPC for tracking fee info once in
 }

lnrpc/rpc.swagger.json

@@ -2633,6 +2633,11 @@
         "median_channel_size_sat": {
           "type": "string",
           "format": "int64"
+        },
+        "num_zombie_chans": {
+          "type": "string",
+          "format": "uint64",
+          "description": "The number of edges marked as zombies."
         }
       }
     },