volser: preserve stats over reclones and restores

volser: preserve stats over reclones and restores

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

Change-Id: I436b39b129b892912533ddeff5569b07224b075c
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>