MINOR: config: add thread-hard-limit to set an upper bound to nbthread

MINOR: config: add thread-hard-limit to set an upper bound to nbthread

On todays large systems, it's not always desired to run on all threads
for light loads, and usually users enforce nbthread to a lower value
(e.g. 8). The problem is that this is a fixed value, and moving such
configs to smaller machines continues to enforce the value and this
becomes extremely unproductive due to having more threads than CPUs.
This also happens quite a bit in VMs, containers, or cloud instances
of various sizes.

This commit introduces the thread-hard-limit setting that allows to only
set an upper bound to the number of threads without raising a lower value.
This means that using "thread-hard-limit 8" will make sure that no more
than 8 threads will be used when available, but it will remain two when
run on a dual-core machine.