Delete the validation thread pool

Delete the validation thread pool

My original intent was "deprecate thread-pool.validation.max," but it
turns out it was just a symptom of a (mostly inoffensive)
overcomplication.

thread-pool.validation.max has proven confusing to users, because it
doesn't make sense for it to be configurable. The thread count should
always equal the number of RPKI trees, which in turn equals the number
of TALs. There's no reason why Fort should offload this decision to the
user.

As for the thread pool, the validation cycle is not really a fitting
problem for such an ellaborate solution, because the former involves a
very small amount (typically 5) of long-lived threads that start at the
same time, once every hour or so.

So instead of pooling a configured amount of threads in the beginning,
spawn raw threads as needed.

The RTR server still employs a thread pool.