eloop: fix eloop object never being freed

eloop: fix eloop object never being freed

We must _never_ take a reference to ourself in a constructor. Otherwise,
the refcnt will be >1 which means if the user calls *_unref() the object
will not get freed.
Therefore, do not add the counter object used for idle sources directly to
the event loop. Instead, add it when the first idle source is registered
and remove it when the last source is removed. This will slightly slow
down performance of idle-sources. However, the whole eloop is not
optimized for speed, yet, so we don't care for now.

Reported-by: Ran Benita <ran234@gmail.com>
Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>