arrow

ARROW-2569: [C++] Improve thread pool size heuristic

The heuristic goes this way:
- if the OMP_NUM_THREADS environment variable exists, it defines the baseline
  number of available threads
- otherwise, the baseline is the value returned by std::thread::harware_concurrency()
- the OMP_THREAD_LIMIT environment variable, if it exists, defined the upper bound
  for the final value, i.e. we return min(baseline, limit), otherwise we just
  return the baseline.

This is the heuristic used by other packages such as the GNU "nproc" utility.

Author: Antoine Pitrou <antoine@python.org>

Closes #2026 from pitrou/ARROW-2569-thread-pool-heuristic and squashes the following commits:

5572b075 <Antoine Pitrou> Factor out environment variable helpers
2dd0d12e <Antoine Pitrou> ARROW-2569:  Improve thread pool size heuristic