[issue30014] Speedup DefaultSelectors.modify() by 1.5x

[Giampaolo Rodola']

Giampaolo Rodola' added the comment:

> The idea is to reuse _BaseSelectorImpl.register() and
> _BaseSelectorImpl.unregister() to factorize the code.

You can't factorize the logic of modify() into those as they do two different things. I also don't like repeating the same thing 3 times but given how the module is organized I'm not sure how to do that as I need to pass 3 things around: the low-level selector (epoll, poll, whatever) and the read and write constants (POLLIN, EPOLLIN) which change depending on the selector being used.
The same thing applies to the devpoll class (http://bugs.python.org/issue18931).
I can write a second patch which to refactor the whole module if that is desirable but I prefer to do that in another PR.

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