Multi-process Mode

[alexfernandez]

Adds the option --cores n to start multiple processes. Shares total requests and requests per second among all available cores, aggregates the results.

Also in this PR:

• Start test server in multi-process mode by default, using half the available cores.
• Start cluster mode using NODE_CLUSTER_SCHED_POLICY, the only way with ESM. Reported issue to Node.js core.* Change values in result: rps is now effectiveRps, totalTimeSeconds is now elapsedSeconds. Old values kept for compatibility.
• Compute elapsed seconds in result using start and end times, with BigInt returned by process.hrtime.bigint().

For immediate merge, to be released as v6.3.0.