lnrpc+rpcserver: enable abandonchannel for externally funded chans

To make sure we can use the abandonchannel RPC for getting rid of
externally funded channels who's funding transaction was never
published, we allow the RPC to be used on non-dev builds for externally
funded and pending channels only.

lnrpc/rpc.proto

@@ -235,8 +235,10 @@ service Lightning {
     /* lncli: `abandonchannel`
     AbandonChannel removes all channel state from the database except for a
     close summary. This method can be used to get rid of permanently unusable
-    channels due to bugs fixed in newer versions of lnd. Only available
-    when in debug builds of lnd.
+    channels due to bugs fixed in newer versions of lnd. This method can also be
+    used to remove externally funded channels where the funding transaction was
+    never broadcast. Only available for non-externally funded channels in dev
+    build.
     */
     rpc AbandonChannel (AbandonChannelRequest) returns (AbandonChannelResponse);
 
@@ -3058,6 +3060,8 @@ message DeleteAllPaymentsResponse {
 
 message AbandonChannelRequest {
     ChannelPoint channel_point = 1;
+
+    bool pending_funding_shim_only = 2;
 }
 
 message AbandonChannelResponse {

lnrpc/rpc.swagger.json

@@ -151,7 +151,7 @@
     },
     "/v1/channels/abandon/{channel_point.funding_txid_str}/{channel_point.output_index}": {
       "delete": {
-        "summary": "lncli: `abandonchannel`\nAbandonChannel removes all channel state from the database except for a\nclose summary. This method can be used to get rid of permanently unusable\nchannels due to bugs fixed in newer versions of lnd. Only available\nwhen in debug builds of lnd.",
+        "summary": "lncli: `abandonchannel`\nAbandonChannel removes all channel state from the database except for a\nclose summary. This method can be used to get rid of permanently unusable\nchannels due to bugs fixed in newer versions of lnd. This method can also be\nused to remove externally funded channels where the funding transaction was\nnever broadcast. Only available for non-externally funded channels in dev\nbuild.",
         "operationId": "AbandonChannel",
         "responses": {
           "200": {
@@ -190,6 +190,13 @@
             "required": false,
             "type": "string",
             "format": "byte"
+          },
+          {
+            "name": "pending_funding_shim_only",
+            "in": "query",
+            "required": false,
+            "type": "boolean",
+            "format": "boolean"
           }
         ],
         "tags": [