MINOR: freq_ctr: unify freq_ctr and freq_ctr_period into freq_ctr

MINOR: freq_ctr: unify freq_ctr and freq_ctr_period into freq_ctr

Both structures are identical except the name of the field starting
the period and its description. Let's call them all freq_ctr and the
period's start "curr_tick" which is generic.

This is only a temporary change and fields are expected to remain
the same with no code change (verified).