A new static member getPreferredConcurrency added to
comphelper::ThreadPool to return a configurable max
number of threads.

By default the new function returns the hardware_concurrency
value provided by std::thread. When MAX_CONCURRENCY envar is
defined, the return value is limited to whatever is set there.

Three call-sites that used std::thread::hardware_concurrency
have been replaced with getPreferredConcurrency.

Unittests added to cover the functionality of the new member.

Unittests are capped to 4 threads.

Change-Id: I3332e393a88a5ed436316fa712ed920a4b37f4af
Reviewed-on: https://gerrit.libreoffice.org/26254Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Ashod Nakashian <ashnakash@gmail.com>