Improve DHCP agent performance

Improve DHCP agent performance

Previously when starting the dhcp agent the sync_state() process would
be extremely expensive as it would query quantum server for each network.
In order to improve performance a get_active_networks_info() was added
so this information could be retrieved in one query rather than doing a
query for each active network.

The second part of this patch optimizes the logic to avoid calling
get_dhcp_port(). Previously, this method was called once for each network
which makes a call to get_subnets() and get_ports() unnecessarily as
the dhcp agent can determine itself if it needs to update a port or create a
port for dhcp.

This patch also threads the inital sync process and maintains backwards
compatibility with the previous rpc api.

There was also a trivial change to the nvp_plugin where filters are assumed to
be a dict.

implements blueprint improve-dhcp-agent-performance

Change-Id: I3b631057f595250dad76516faa9b421789f60953