Refactor process_router method in L3 agent

Refactors the process_router method of the L3NATAgent class to several methods
that coherently group functionallity. The aim is to make the code more readable
and maintainable. There is no functional change in this patch.

Partially-Implements: bp restructure-l3-agent

Change-Id: Id35a5f10cd7d2dda565bffb14fc6a2fbd7eef3ea
Merge "Refactor process_router method in L3 agent"