auth: avoid excessive stat of cellservdb

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.

Change-Id: I7d9a58f9af72b8c36b50374e49c07b4db85285e0
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>