Phantom column in library table ---(Bug #1258776) fixed

KEY_ID is only needed to cache the global key and like all internal
housekeeping variables in the database it is internal.
Merge branch 'master' of https://github.com/mixxxdj/mixxx into osx_build_fixes
Remove unnecessary code which breaks compile on other than linux.

The thread cpu timer is adopted from performance timer code for
linux. There is no implementation for other platforms, but the old
functions were left around leading to duplicate symbols when linking.

Removed these and renamed 'default' dummy functions.
Fix build on windows

Readded ther windows performance timer. This now compiles but
I haven't tested if it actually works.