appropriate

Rename ThreadPool -> FixedSizeThreadPool to allow for later introduction
of DynamicSizeThreadPool

This reverts commit 5d5b1d8c8ec4c578f72a12f5b81f705764cdf804.

And make it easy not to make the same mistake

- notify_one when new work is available to avoid thundering herd
  problems
- checking conditions before entering cv_.wait() prevents a case that
  these are not validated