Set route priority lower than controller traffic

Previously, we had a single base priority which was used for route
priorities (and modified for LPM), and also for controller traffic. This
meant that traffic bound for the default route could match before
controller-bound traffic. This patch fixes the issue by introducing four
priority bands:

PRIORITY_LOWEST: 0
PRIORITY_LOW: 16400 (For routes, adjusted by +10*Prefix length)
PRIORITY_HIGH: 32800 (For controller traffic)
PRIORITY_HIGHEST: 49200 (Unused, but could implement firewall-style rules)

One side effect of this adjustment under the current codebase is that
all controller-type traffic (ALL BGP,ICMP,ARP,LDP,OSPF,etc) will be
routed via the controller. A future patch should adjust this to restrict
such traffic to only controller-type traffic that is destined for the
controller.

This fixes issue https://github.com/joestringer/RouteFlow/issues/2.

Signed-off-by: Joe Stringer <joestringernz@gmail.com>