Refactor: Consolidate Transaction and Publish Handling

This consolidates the transaction and publish handling in order to make it more robust for failing remounts. What is still missing is an automatic recovery functionality to get out of error states.

**Note:** This contains [Pull Request 539](https://github.com/cvmfs/cvmfs/pull/539) and [538](https://github.com/cvmfs/cvmfs/pull/538).
Here, use_fd_fallback is always zero, no?  Shouldn't the same double check be done as for the publishing?