LISP: Add authenticated MReg perf test

In addition to the unauthenticated Map-Register test, add an
authenticated Map-Register test, which is a more real-world scenario,
and it also shows if authentication has a significant impact on
performance.

While at it, do some refactoring to make the code more readable and
reusable.

Change-Id: I9cc9cdcd0ca0ae6777fea66a3686997f5dca07bc
Signed-off-by: Lorand Jakab <lojakab@cisco.com>