Made something of a mess, but multi-threading is enabled for stage 2 and 3.
Multi-threading stage 1 isn't worth it (0.5 seconds for 37746 files - that scales well).
The MD5 I am using is not thread safe, so my times when all the blocks in in my
/usr/local are cached are only slightly better, but it appears my "cold start"
times are much better. I found fdupes on the web today, but I haven't looked at
it yet. I want to "do the right thing" for hard links and then clean this up.
A thread safe MD5 would be nice and will probably be the next step after hard links.
After I begin to like this I may test it against fdupes.