Now the epoll instance of the Worker is used when polling. The
work is still done in poll.cc and the worker provides the descriptor
and thread id.
The comments of poll_waitevents() have been removed; they were not
accurate anymore so better to let the code speak for itself.
The shutdown is now performed so that a shutdown message is
sent to all workers. When the workers receive that message, they
turn on a shutdown flag, which subsequently is checked in the poll
loop.
