Add flag back to disable individual thread pools

In 4.0, xxx_thread_pool.enbled was removed. The idea was that this could be done programmatically by using TP.setOOBThreadPool() using a DirectExecutor.
However, sometimes it is convenient to do this declaratively, in the XML config file, e.g. if the code cannot be changed or the channel cannot be retrieved.

Therefore add the attribute(s) to disable/enable thread pools back.