Fix bugs in ThreadPoolSvc

Fix bugs:

• avoid loosing one worker thread in the TBB global thread pool when ThreadPoolSvc is configured to use all available hardware threads;
• initialize boost::barrier with correct counter to avoid leaving one ThreadInitTask in limbo at the termination of ThreadPoolSvc (this revealed itself as a deadlock with changes of !1067 (merged));
• make ThreadPoolSvc set the right number of worker threads in Gaudi::Concurrency::ConcurrencyFlags.

Also:

• remove unused headers and narrow down used ones in ThreadPoolSvc;
• change the AvalancheSchedulerSvc's help message about worker thread pool size to reflect what is actually going on in the ThreadPoolSvc.

added bug label

changed the description

assigned to @clemenci

changed milestone to %v33r2

/ci-test --merge

• [2020-05-20 09:41] Validation started with lhcb-master-mr#826

resolved all threads

approved this merge request

mentioned in commit c7d6fdfd

merged

mentioned in issue #132 (closed)