Move classes out of l3_agent.py

The file l3_agent.py has become too large.  This patch is a simple
pure refactor to move some of the functionality in to other files
where things aren't too tangled up.  There is no functional change
with this patch and I avoided gratuitous other fixups in this patch in
order to make it easier to review.

I plan to follow up on the new l3_dvr and l3_agent_router modules with
more restructuring in the near future.

Partially-Implements: bp restructure-l3-agent

Change-Id: I3529fe4146c50c940f41eb26d0b5efc5870b3af9
Merge "Move classes out of l3_agent.py"