perf: add aggregator helper for bench statistics

This patch adds a helper script to aggregate the benchmark results from
JSON files to the format parsable by the InfluxDB line protocol [1].

All JSON files from the <perf/output> directory are benchmark results
and aggregated into the <perf/output/summary.txt> file that can be
posted to the InfluxDB. The results are aggregated via the new target
test-perf-aggregate, which is run only if some JSON files with results
are missed.

[1]: https://docs.influxdata.com/influxdb/v2/reference/syntax/line-protocol/

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