Multi-threaded stats

[randy408]

Describe the problem you are trying to solve.
The stats addon and explorer is not MT aware, the values stop making sense with multiple threads.

Describe the solution you'd like
Add support for per-thread stats, thread-specific stats (could indicate how well work is distributed, etc), and proper sums for time spent in systems (wall clock vs cpu time). And then support for it in explorer, of course.

[ZeroErrors]

To add a little more detail to this issue.

Currently the system_time_total is calculated only on the main thread:

flecs/src/addons/pipeline/pipeline.c

Lines 632 to 641 in b5241cc

	if (measure_time) {
	ecs_time_measure(&st);
	}
	const int32_t i = flecs_run_pipeline_ops(world, stage, stage_index, stage_count, delta_time, true);
	if (measure_time) {
	/* Don't include merge time in system time */
	world->info.system_time_total += (ecs_ftime_t)ecs_time_measure(&st);
	}

This means that the stats only knows about the amount of time the operation took to execute the main thread portion of the work, however, systems may have unbalanced work. Meaning a worker thread could have more to do than the main thread. If this happens it will cause the time interval on the main thread will not be equivalent to the amount of time the system actually took to execute.