volser: preserve stats over reclones and restores

Optionally, preserve the volume usage statistics instead of clearing
them during reclone and restore operations.

Reviewed-on: http://gerrit.openafs.org/3312
Reviewed-by: Alistair Ferguson <alistair.ferguson@mac.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit dfceff1d3a66e76246537738720f411330808d64)

Change-Id: I2e47c7ddac74f75b6f650327028644e178cfc574
Reviewed-on: http://gerrit.openafs.org/9477
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>