Before this change the union remote was using whether the writable
union could poll for changes to decide whether the union mount could
poll for changes.
The fix causes the union backend to signal it can poll for changes if
**any** of the remotes can poll for changes.
Implement optional interfaces
- Purge
- PutStream
- Copy
- Move
- DirMove
- DirCacheFlush
- ChangeNotify
- About
Make Hashes() return the intersection of all the hashes supported by the remotes
* Fix error handling in List and NewObject
* Fix Precision in case we have precision > time.Second
* Fix Features - all binary features are possible
* Fix integration tests using new test facilities