feedback_daemon: add operation statistics reporting

Report box.stat().*.total, box.stat.net().*.total and
box.stat.net().*.current via feedback daemon report.
Accompany this data with the time when report was generated so that it's
possible to calculate RPS from this data on the feedback server.

`box.stat().OP_NAME.total` reside in `feedback.stats.box.OP_NAME.total`, while
`box.stat.net().OP_NAME.total` reside in `feedback.stats.net.OP_NAME.total`
The time of report generation is located at `feedback.stats.time`

Closes #5589