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 0e3bfa033ed230fcb46ad8e3c26c8b7aae6e00af

Change-Id: I209e93a1bc4107a878eefaae92ec0e5e4ada2518
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>