This puts ThreadPool::join() back to being private.
Eventia events now should inherit from Event, which will take
care of setting up the required callbacks so that events will
be notified about the main event loop stopping. Inheriting
from Event is currently not enforced.
This splits the thread pool code from the event code, and
class names are now wrong (I will update them in the next
commit). Also, this disregards the roar11 vs tf library
thing, taskflow was not working anyways and I will probably
remove all the conditional compilation stuff about it for
now.
