If we open a file for r/w without the cache we now always return a
handle and return an error if the file is ever read from. This fixes
incompatibility with cmount under windows.
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)
* Tests for VFS layer
* Small fixes found during testing
* Fix Close, Flush and Release behaviour for ReadFileHandle and WriteFileHandle
* Fix nil object bugs on File