auth: avoid excessive stat of cellservdb

The modified time resolution is one second, so only
stat the cellservdb file at most only once per second.

Reviewed-on: http://gerrit.openafs.org/5743
Reviewed-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@secure-endpoints.com>
(cherry picked from commit 0e3bfa033ed230fcb46ad8e3c26c8b7aae6e00af)

Change-Id: Ia533d037cc772c085cfaad47035e2a29df64952e
Reviewed-on: http://gerrit.openafs.org/9435
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>