Use Netty Epoll in SB when available

Use Netty Epoll in SB when available

Preliminary performance testing shows that using Epoll on Linux x64
increases Map-Request, un-authenticated Map-Register and authenticated
Map-Register response rates from an average of 57k, 44k, and 31k replies
per second respectively to 88k, 55k, and 38k replies per second on our
internal reference testing platform.

Note that the sending rate is 100k requests per second, which means that
at higher sending rates the Map-Request performance may be even better
(although still "lossy").

This is the first step in improving southbound performance, with a
second patch to follow for multi-threaded packet handling, which depends
on Epoll (thus this patch).

Change-Id: I540a755850fb4c40ed0b1a4c745dbb4a5fa7bc9f
Signed-off-by: Lorand Jakab <lojakab@cisco.com>