bev_async: trigger/run only deferred callbacks

bev_async: trigger/run only deferred callbacks

Otherwise callbacks will be runned even without event_loop, due to
nature of IOCP.

A simple example is:
  evhttp_connection_free(client)
  # freeing the client will trigger evhttp_connection_free() for the
  # client on the server side, and hence there will double free
  evhttp_free(server)

Fixes: iocp/http/simple