This was causing errors if the cache cleaner was called between the
Open and the pendingOpen of a RW file.
The fix was to move the cache open to the Open from the openPending.
This adds new flags to mount, cmount, serve *
--cache-max-age duration Max age of objects in the cache. (default 1h0m0s)
--cache-mode string Cache mode off|minimal|writes|full (default "off")
--cache-poll-interval duration Interval to poll the cache for stale objects. (default 1m0s)