Introduce a handler that lives under the gorilla mux and adds the
correct headers based on the request and always returns 200 OK when
there OPTIONS is called on a URL.
This fixes the ability to DELETE from the dashboard on peer X when peer
Y is the leader. As a side effect it reveals some bugs in the dashboard
though notably:
- Due to the RTT immediatly refreshing the dashboard doesn't work and
deleted keys are still there
- For some reason PUTS from peer X are creating directories and not
keys.
