Speed up initial L3 full sync time

Speed up initial L3 full sync time

When a L3 agent starts up the initial full sync occurs
on the first periodic task interval.
This means that from the point the agent is ready to
process updates it can take ~1 minute until traffic
can start flowing through the L3 agent's routers.

When using a highly available solution (not VRRP)
the startup delay adds around a full minute to
the minimum downtime.

This patch simply does a manual (i.e. not periodic) full
sync once the L3 agent is ready (in 'after_start' method).

The change results in a much faster sync on a new agent.
With the patch it takes several seconds until routers are
created on the new/restarted L3 agent.

Closes-Bug: #1407410

Change-Id: I2447a4cdfff5e915f0b5da88e16c9b6944ea9563