I don't think it's safe to use map.iteritems() in the various poll

I don't think it's safe to use map.iteritems() in the various poll
routines.  I got some errors "dictionary changed size during
iteration" when running ZEO tests on machine while doing heavy
forground work in another window, and thinking about it, I believe
that it should be okay if readable() or writable() modifies the map.

I also finally made all the spacing conform to the Python style guide:
no space between a function/method name and the following left
parenthesis (fixed lots of occurrences), spaces around assignment
operators (fixed a few, always of the form "map=..."), and a blank
line between the class statement and the first method definition (a
few).