Allow cherrypy users to pass in a custom poller to the manager

[adamcik]

While digging into mopidy/mopidy#445 I found that the manager class allows for custom pollers, a feature we would like to use to integrate ws4py with our main gobject event loop used by the rest of our server.

However, ws4py/server/cherrypyserver.py doesn't allow for passing it in short of monkeypatching or sub-classing the WebSocketPlugin to do my own manager initialization. Please consider making this easier to achieve :-)

[adamcik]

I've actually oversimplified this a bit given the use case I mentioned. Passing a different poller in is sufficient to work around things like #94 without having to wait for it to be fixed (though just having this fixed in ws4py is clearly preferable).

Other case I was thinking of, using our GObject main loop via https://developer.gnome.org/pygobject/stable/glib-functions.html#function-glib--io-add-watch would not cleanly integrate with the current code. The entire manager would likely need to be replaced with one that doesn't use threads and just lets the callbacks do their thing, or the code could be re-factored to move the loop in run into a thread outside of the manager, and then make it up to the poller if a thread is needed or not. Basically just having that thread emulate the callbacks so the rest of the code behaves in the same way.

Any way, this is mainly intended as input on one way to maybe extend the API to allow for a wider array of pollers to be plugged in cleanly. With this in mind looking at the work done in tullip and PEP3156 could likely be an even better way forward than what I've suggested.

[Lawouach]

The best course of action is indeed support of PEP3156. I don't think ws4py should reinvent too much the wheel when it comes to event loop. I sometimes believe we could rely on Tornado entirely, but then what would be the point of ws4py itself as one might as well simply use their websocket implementation.

This is why I like PEP3156 indeed, delegate the loop to an external provider.

[Lawouach]

I'm going to close this one for now as I'd rather focus on moving to PEP3156. This could be reopened if need be.