vos: Minimize release impact for new RO sites

Currently, if a new RO site is added with 'vos addsite', the only way
to populate the new site with data is a 'vos release' (excepting hacks
using 'vos restore' and 'vos addsite -live', etc). Due to safeguards
in 'vos' ensuring that RO sites always all contain the same data when
marked as up-to-date in the VLDB, such a release always incurs some
amount of data to be transmitted to all sites, as well as remote sites
being brought offline briefly, even when the RW data has not changed
in very long time.

To alleviate this situation, make 'vos release' detect if new,
unpopulated RO sites have been added, and if the RW volume has not
changed since the release of any existing RO sites. If both of these
conditions are true, do not update any of the existing sites, but only
transmit volume data to the sites that did not already contain RO
volumes.

Reviewed-on: http://gerrit.openafs.org/7393
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit 3fc800be9c702c1a40869908831a9895602909cb)

Change-Id: Icfeb1bad816ff5c3220581c7adf541d8b87df989
Reviewed-on: http://gerrit.openafs.org/10174
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>