Save frequencies when bulk-saving of times is enabled

[oleksandr-pavlyk]

Closes #318

This PR:

• applies changes started by @gevtushenko to collect bulk values of frequency estimates and enable saving them alongside bulk of time-stamps
• the same batched writeout_data utility is used to save duration and frequency data faster

[oleksandr-pavlyk]

Questions to be answered:

• should dumping of frequencies require a CLI option separate from the one controlling dumping of time-stamps?
  • Frequencies are saved when throttling control is enabled.
    Q: what happens when throttling control is turned off and bulk data is requested?