Refactoring of the Asynchronous Reader

This factors out the Asynchronous Reader used in the new TBB backend to make it usable for the upcoming `swissknife scrub` tool. Additionally it adds some unittests targeted to the Asynchronous Reader.

**Note:** re-open due to space-time discontinuity

**Note:** this contains [Add Unittests for new components](https://github.com/cvmfs/cvmfs/pull/245)
**Note:** this contains [Integrate the TBB based file processing into the Spooler](https://github.com/cvmfs/cvmfs/pull/244)
**Note:** this contains [New File Processing Pipeline based on TBB](https://github.com/cvmfs/cvmfs/pull/243)