MINOR: haload: add rate limiting support using -R option

MINOR: haload: add rate limiting support using -R option

Implement request/connection rate limiting per second using the new
-R command line option <arg_rate>.

Instead of reinventing the wheel, this implementation aligns with the
h1load approach by relying on existing frequency counters. Dedicated
tasks <hld_rate_task> are spawned on each thread solely to manage and
throttle connection initializations based on <arg_rate>.

The counter <running_usrs> is renamed to <running_tasks> to track both
users and connection rate-limiting tasks.

Modify the doc/haload.txt file.