lisa.trace: Use category dtype for some columns

Use category dtype for "__comm" and "comm" columns, since they are
heavily used strings. This allows saving a lot of memory, at the
expense of groupby() speed (x10 slowdown). The filtering speed is a bit
improved (x1.1 speed up)

sched_switch dataframe also uses category for prev_comm and next_comm
columns, achieving a reduction of 5x in memory consumption.