skip progress output if another thread is printing

With a large number of threads, progress output becomes increasing spammy and
contention on the print lock can begin to degrade runtime. Since progress output
itself is non-critical, simply suppress it when the print lock is held by
another thread.

It's no longer relevant to experiment with replacing this with a spinlock. Some
cursory experiments indicated this did not affect performance on mid-size
machines. On large machines a spinlock is likely to perform worse than a pthread
mutex, that we expect has already been optimised for large machine scenarios.

Github: closes #160 "replace print mutex with a spinlock?"