unitrack 6.x-1.5

Maintenance release focused on performance.
Removed unused tracking code, potentially slow db calls moved to shutdown functions to serve client as quick as possible. Added updated column to unitrack_visitors table, which significantly sped up clean up job.
Most chances this is the last stable release in 6.x branch.
From now on, development efforts will be focused on stable 7.x branch. Version 1.6 will be (hopefully) released in 7.x, and then ported to 6.x if necessary.