This patch adds ctime/mtime support to the “main” config objects - the
config data itself, and the cluster/nodes/instances objects.

These are not added on auto-upgrade, but rather should be migrated if it
makes sense when 2.0 is upgraded to 2.1.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>