common: never remove forcibly a reused fd from epoll.

common: never remove forcibly a reused fd from epoll.

If during a single iteration of epoll event dispatching
a file descriptor is closed and reused, make sure the
new reused file desciptor is not accidentially removed
from epoll.

During dispatching we try to patch things up to prevent
a busy looping flood of EPOLLRDHUP/EPOLLHUP events for
users that don't properly handle them. This bug was
triggered when the closed file descriptor got reused and
a new watch for it readded from within the I/O watch
callback for the HUP event on the original closed descriptor.

This patch also makes sure that an effectively similar bug
can not be triggered by calling mrp_del_io_watch twice for
the same I/O watch.