Periodic ARP resolver should use unicast MAC destination address whenever possible; instead of broadcast

As a continuation to the work of OVSDB-160 the periodic 10 seconds
ARPs should be using unicast – instead of broadcast – whenever possible.

Using broadcast unconditionally works, but causes unwanted flood of arp
queries every interval. Implementations of similar functionality switch
to use the known unicast mac as destination while unchanged answers are
coming back from the ip address being monitored.