Fix the comparator used for sorting rules/classifiers

On some (old?) gcc compilers the earlier comparator function
was getting discarded (possibly due to optimizations). This would
causes unexpected bad-memory access during sort or wouldn't sort
the vector at all. The crash/sort-issue were easily reprodubile on
a CentOS 6.5 system with gcc 4.4.7 compiler. Defining a comparator
class seems to fix both issues on this system - was able to run 1000
iterations of "make check" successfully.

Change-Id: I93af90e8549a7b56a5c118faf46180201b2ebba1
Signed-off-by: Amit Bose <bose@noironetworks.com>