use wait/signal with pthread_cond_wait to wake up workers

[boisvert@bigmem biosal]$ time ./scripts/development/run-argonnite-4-7.sh | tee log
real    30m46.229s
user    713m5.749s
sys     52m9.993s
[boisvert@bigmem biosal]$ sha1sum output/coverage_distribution.txt-canonical
71deaa88265222cd8c27c88980eb5c4f29c966af  output/coverage_distribution.txt-canonical

thorium events for a computation:

[thorium] node/2 COMPUTATION LOAD 1847 s 5.16/6 (0.86) 0.85 0.86 0.88 0.86 0.86 0.85
[thorium] node/2 COMPUTATION WAKE_UP_COUNT 1847 s  96113 108078 74854 89107 87257 102198

thorium events for an epoch:

[thorium] node/3 EPOCH LOAD 1845 s 5.93/6 (0.99) 0.99 0.99 0.99 0.99 0.99 0.99
[thorium] node/3 EPOCH WAKE_UP_COUNT 1845 s  1 1 3 3 1 1

Link: https://github.com/GeneAssembly/biosal/issues/470
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>