Improve DVR scale performance

Improve DVR scale performance

Only process floating ips on a router that are relevant to the agent
hosting the router (don't process floating ips assigned to a router
if the associated vm is not hosted on the compute node requesting the
router sync). In this way, the number of database calls made during
the DVR router updates is optimized to eliminate unnecessary
duplication of calls which return the same data or are made to get
data for routers which are not relevant to the sync_routers request
from the agent.

Change-Id: I4e8477bb61ffff164d2f3bbebb94e95a25838ce0
Partial-Bug: #1413314