perf: standardize 1mops_write benchmark

The (not commented) output now contains average items per second instead
of peak speed.

Also, two new options are introduced:
* The `--output` option allows you to specify the output file.
* The `--output_format` option means the format for the printed output.
  The default is "console". It just prints inserts per second value to
  the stdout. The "json" format contains all the information about the
  benchmark in a format similar to Google Benchmark's.

Usually, these options should be used together to dump machine-readable
results for the benchmarks.

The test can still be run standalone to be usable alongside Tarantool's
repository.

NO_DOC=perf test
NO_CHANGELOG=perf test
NO_TEST=perf test