try to fix "too many open files" error

asyncutils.nim

@@ -39,9 +39,13 @@ proc asyncExecCmd*(command: string): Future[string] =
   let flowVar = spawn execCmdBackground(successEvent, failureEvent, command)
   proc successCallback(fd: AsyncFD): bool =
     future.complete(^flowVar)
+    successEvent.unregister()
+    failureEvent.unregister()
     true
   proc failureCallback(fd: AsyncFD): bool =
     future.fail(newException(OSError, ^flowVar))
+    successEvent.unregister()
+    failureEvent.unregister()
     true
   addEvent(successEvent, successCallback)
   addEvent(failureEvent, failureCallback)