Bootstrap thread safety

Changed options and attrs from LinkedHashMap to ConcurrentHashMap to
avoid a possible ConcurrentModificationException if a thread was
adding/removing options/attrs while another was calling Bootstrap.init()
or Bootstrap.duplicate()

Looking at the rest of Bootstrap I think this should make it threadsafe re: creating new channels? The factories and such are immutable so it seems like this would do it. I also don't see anything that seems to warrant the preserved ordering with either options or attrs ?