Whitespace fixup
Comment fixup
Problem: expiry timers are not scalable for many clients

See https://gist.github.com/hintjens/795ddedd577e3e0f00fc

Solution: use tickets instead of timers. I've not tested the
performance change in zproto_server yet, just simulated it with
the above test case, and ensure that nothing has broken so far.

I'd be very interested in any results from people with the
infrastructure to test hundreds or thousands of clients.
Merge pull request #157 from hintjens/master

Problem: zloop timers do not scale nicely