Varnish tuning for vic

OK this does a few things:

* Added a flag to the corsProxy obect.  If present it appends the flag to /proxy/... to create /proxy/_flag/... 
* This is called with a '1d' flag in GeoDataCollection when doing getCapabilities, so that we update the proxy cache every 24 hours.  Otherwise urls are cached for 2 weeks.
* Server.js peels off the flag from the remoteUrl.

The flag right now is just a placeholder (it can only set the ttl to 1d), but we can use this mechanism for more interesting proxy communication in the future if we want.