HPCC-11358 Sort WUs by Total Thor Time for both 5.0 WUs and 4.x WUs

The Total Thor Time of 5.0 WUs is stored under Statistics/Statistic
[@desc='Total thor time']/@value. For 4.x WUs, it is under Timings/
Timing[@name='Total thor time']/@duration. To sort the WUs, this
fix creates a multivalue sort key for the Total Thor Time. The
multivalued sort key is preparsed by the new code in this fix.
Now, WUQuery can sort WUs by Total Thor Time for both 5.x WUs and
4.x WUs.