[MINOR] freq_ctr: add new types and functions for periods different from 1s

[MINOR] freq_ctr: add new types and functions for periods different from 1s

Some freq counters will have to work on periods different from 1 second.
The original freq counters rely on the period to be exactly one second.
The new ones (freq_ctr_period) let the user define the period in ticks,
and all computations are operated over that period. When reading a value,
it indicates the amount of events over that period too.