Use a UNIX timestamp in the LoggedOut cookie

Since it is easier to interpret in VCL. The read side code strangely
does not need to be updated, and MW will continue to work with
cookies sent before this change, because wfTimestamp() automatically
detects the timestamp format.

Change-Id: I8c661b735b1de8e295d5d713a5ca959ee99e274e
Merge "Use a UNIX timestamp in the LoggedOut cookie"