TorClient: use thread-safe collection for guards

This commit changes the list type used for storing guard that
need to be destroyed to a thread-safe option this removes the
need for locking the list which according to experience causes
it's own problems.

NOnion/Client/TorClient.fs

@@ -1,6 +1,7 @@
 namespace NOnion.Client
 
 open System
+open System.Collections.Concurrent
 open System.IO
 open System.Net
 open System.Net.Http
@@ -72,8 +73,7 @@ type TorClient internal (directory: TorDirectory) =
             return new TorClient(directory)
         }
 
-    //FIXME: lock this
-    let mutable guardsToDispose: List<TorGuard> = List.empty
+    let guardsToDispose = ConcurrentBag<TorGuard>()
 
     static member AsyncBootstrapWithEmbeddedList() =
         async {
@@ -124,7 +124,7 @@ type TorClient internal (directory: TorDirectory) =
                                 ipEndPoint
                                 (nodeDetail.GetIdentityKey() |> Some)
 
-                        guardsToDispose <- guard :: guardsToDispose
+                        guardsToDispose.Add guard
                         return guard, nodeDetail
                     with
                     | :? GuardConnectionFailedException ->