[#2310] Fix executeReactor to wait for threadpool.

Problem
=======

executeReactor does not wait for thread from threadpool


Why we got this
============

It looks like thread from threadpool are different from the reactor own threads... most probably created while Twisted has evolved and own threads are only used in legacy code.

We had no test to use threads from threadpool.


Changes
=======

Write a test and fix the problem
Reduce the step interval in debug mode from 1 second to 0.1
Update debugging information with status about threadpool.

How to test
=========

reviewers: @alibotean 

check that changes make sense.

Thanks!
Update docstring (1 second  -> 0.1 seconds)