Add a new Poller implementation that uses epoll rather than select

This should scale to a large number of FDs. I still need to do some benchmarking. 

epoll can be enabled with the --use-epoll flag. Additionally for tests setting OLA_USE_EPOLL=true in the environment will enable it. 


Closes #229 
I wrapped the flag in the #ifdef so it doesn't show up unless the platform supports epoll.  It's exported on /debug as well under use-epoll.