[REQ]Handle cases where hardware_concurrency() returns 0

[bstamour]

std::thread::hardware_concurrency should only be treated as a hint

C++'s std::thread::hardware_concurrency() is allowed to return 0 if it cannot compute the value for you. From a cursory reading of the code: if this were to happen, you will have an empty thread pool. It may be better to use a value of 1 in that case, so that you can still submit work.

[bshoshany]

Thanks, this will be fixed in the next release (see #51).