Add ScannerTask, ScannerGlobal, ImportFilesTask and RecursiveScanDirectoryTask.
Parallelize the recursive scan step of the library scanner.

Uses 10 threads currently.
Add benchmark TODO.
Address review comments.
Use a single scanner thread to start.
Merge pull request #382 from rryan/scanner

Parallelize recursive scan phase of the library scanner.
Update LateNight and Deere equalizer rack parameters to go high to low.

See discussion in Bug #1395285.
Fix subtle issue with playlist import ordering. Fixes Bug #1241244.

TrackDAO::addTracks does two passes over the imported tracks list -- the
first collects track IDs which are already in the library. The second
adds tracks not in the library to the library. I added a third step to
re-sort the tracks by the caller's provided ordering. Includes some
other misc fixups.
Work in progress...