FIX: Double Check for Open File Descritpors in `rollback()`

This performs a similar file descriptor double-check in `rollback()` as [previously intruduced](https://github.com/cvmfs/cvmfs/pull/538/files) into `publish()`.

Note: This contains the following Pull Requests:
[Refactor: Consolidate Transaction and Publish Handling](https://github.com/cvmfs/cvmfs/pull/552)
[FIX: Check Mounted Root Catalog Hash Early](https://github.com/cvmfs/cvmfs/pull/539)
[FIX: Remount With Open File Descriptors](https://github.com/cvmfs/cvmfs/pull/538)