auth: fix cellservdb update check

Fix a bug introduced by the check to avoid excessive stats of the
cellservdb.  Fixes a bug where cached cell config data is served for up
to one second after a write.

Check the timeRead field which is reset after a write to indicate the
data should be read.

Fixes commit 5cdc211b8bf1b3f5419ebe52ecc9d3772ffdf9a6 on the 1.6.x branch.

Reviewed-on: http://gerrit.openafs.org/10230
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>

(cherry picked from commit 73ae85bd59cfe9fca2daeb7dc0aa0917368bb5bf)

Change-Id: I01c09383b0ba417aca13b2ef7724d4fde91e327a
Reviewed-on: http://gerrit.openafs.org/10247
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>