Consolidate unused total_width and the way avg_width is computed in PerColumnStats

PerColumnStats.total_width is an unused field in incr-stats-util.cc while the comment says that it is the result of (avg_width * num_rows). Later similar multiplication can be found in the Update method, however the result is stored in avg_width.

avg_width += (new_avg_width * num_new_rows);

We should consolidate these fields and the calculations.