volser: preserve volume stats by default

volser: preserve volume stats by default

Commit dfceff1d3a66e76246537738720f411330808d64 added the
-preserve-vol-stats flag to the volume server. This enabled a change in
the volume server to preserve volume usage statistics during reclone and
restore operations. Otherwise, volume usage counters of read-only
volumes are cleared when volumes are released, making it difficult to
track usage with the volume stats.

Make this feature the default behavior of the volume server and provide
the option -clear-vol-stats to use the old behavior if so desired.  This
change makes the -preserve-vol-stats the default, and keeps it as a
hidden flag for sites which may already have that flag set in the
BosConfig.

Since this changes a default behavior of the volume server, this change
is only appropriate on a major or minor release boundary, not in the
middle of a stable series.

Change-Id: I3706ede64b7b18a80b39ebd55f2e1824bb7dbc57
Reviewed-on: https://gerrit.openafs.org/12674
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>