MEDIUM: tasks: implement a lockless scheduler for single-thread usage

MEDIUM: tasks: implement a lockless scheduler for single-thread usage

The scheduler is complex and uses local queues to amortize the cost of
locks. But all this comes with a cost that is quite observable with
single-thread workloads.

The purpose of this patch is to reimplement the much simpler scheduler
for the case where threads are not used. The code is very small and
simple. It doesn't impact the multi-threaded performance at all, and
provides a nice 10% performance increase in single-thread by reaching
606kreq/s on the tests that showed 550kreq/s before.