chore(benchmark): clear stale benchmark data (#763)

* chore(benchmark): clear stale benchmark data

Originally, benchmark util merge benchmark data, i.e. only inserted new
data in the benchmark store.  However, sometimes benches disappear, and
incorrect information is stored for them (i.e. AquaVM version which is
per-machine).

However, sometimes this functionality is useful, when you want to add a
new benchmark without running all the others.  For such case,
`aquavm_performance_metergin run --unsafe-merge-results` option is
added.

* Restore the `null` benchmark

* Do text report generation exactly one time

It was regenerated from 0 to N times before, and it is wrong.