Thread pool: Convert threads to joinable

Thread pool: Convert threads to joinable

Hypothesis: Something (which I haven't spotted yet) was causing the
main thread to skip its wait before the pool threads finished their
tasks. Maybe something to do with the ready signal again?

So the main thread returned early, which means pool threads were
silently suppressed by the OS. That explains the early terminations and
nonexistent stack traces.

If I keep finding crippling errors like this, I will definitely have to
purge the thread pool. It's turned out to be a fucking bug colony at
this point. I'm sick of it.

Way I see it, the root of the problem was the thread pool's control
code, which was too complicated for its own good. A surprisingly large
part of why it was overcomplicated was because it reinvented thread
joining.

So I simplified the control code by removing the detach property. Now
that the main thread joins the proper way, the validation code will not
be interrupted anymore.

This might well be the solution for #49. However, it bothers me that I
still don't have a reasonable explanation as to why the main thread
seemed to be skipping wait.

Argh!