ubik: check if epoch is sane before db relabel

ubik: check if epoch is sane before db relabel

The sync-site relabels its database at the end of the first write
transaction. The new label will be equal to the time at which the
sync-site in question first received its coordinator mandate. This time
is stored by a global called ubik_epochTime. In order to make sure that
the new database label is sane, only relabel the database if
ubik_epochTime is within a specific range.

Change-Id: I2408569e5de46d387f63cbc2fab05ea1264a505c
Reviewed-on: https://gerrit.openafs.org/12640
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Andrew Deason <adeason@dson.org>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>