lnd: make closechannel send an instant empty update when no_wait set

We do things this way to keep behavior consistent across REST, gRPC
and CLI consistent. This was done to not alter the way we handle
Recv calls from the streams.

lnrpc/lightning.proto

@@ -2046,12 +2046,18 @@ message CloseChannelRequest {
     //
     // NOTE: This field is only respected if we're the initiator of the channel.
     uint64 max_fee_per_vbyte = 7;
+
+    // If true, then the rpc call will not block while it awaits a closing txid.
+    // Consequently this RPC call will not return a closing txid if this value
+    // is set.
+    bool no_wait = 8;
 }
 
 message CloseStatusUpdate {
     oneof update {
         PendingUpdate close_pending = 1;
         ChannelCloseUpdate chan_close = 3;
+        InstantUpdate close_instant = 4;
     }
 }
 
@@ -2060,6 +2066,9 @@ message PendingUpdate {
     uint32 output_index = 2;
 }
 
+message InstantUpdate {
+}
+
 message ReadyForPsbtFunding {
     /*
     The P2WSH address of the channel funding multisig address that the below

lnrpc/lightning.swagger.json

@@ -866,6 +866,13 @@
             "required": false,
             "type": "string",
             "format": "uint64"
+          },
+          {
+            "name": "no_wait",
+            "description": "If true, then the rpc call will not block while it awaits a closing txid.\nConsequently this RPC call will not return a closing txid if this value\nis set.",
+            "in": "query",
+            "required": false,
+            "type": "boolean"
           }
         ],
         "tags": [
@@ -4367,6 +4374,9 @@
         },
         "chan_close": {
           "$ref": "#/definitions/lnrpcChannelCloseUpdate"
+        },
+        "close_instant": {
+          "$ref": "#/definitions/lnrpcInstantUpdate"
         }
       }
     },
@@ -5236,6 +5246,9 @@
       ],
       "default": "INITIATOR_UNKNOWN"
     },
+    "lnrpcInstantUpdate": {
+      "type": "object"
+    },
     "lnrpcInterceptFeedback": {
       "type": "object",
       "properties": {