The previous code did a cheap hash based on pointers, under the
assumption that the IP configs don't get recreated.  But with IPv6
the IP6 config that's eventually applied is a composite of the
DHCPv6 and the RA information, and is thus recreated each time
something in the DHCPv6 or RA changes. Switch to actually hashing
the IP config data and its order to prevent this problem.

Next, add functions to signal that a batch of updates will be
started, and to only commit those updates when all of them
have landed, and if they have actually changed anything.  We'll
use these functions later to reduce the number of changes
that get made to /etc/resolv.conf.