perf: standardize gh-7089-vclock-copy benchmark

The output now contains items per second without the mean time in
seconds. The number of iterations is reduced to 40 to avoid running the
test too long. The `wal_mode` option (default is "none") is set via
command line flags, as far as the number of nodes (default is 10). Also,
the master nodes are set up via the `popen()` command without using any
Makefile.

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 prints items proceeded per second 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.

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