Change behaviour of set_data_pool expiration

The api was somewhat inconsitent regarding set/update of expiration
date and was missused in several places. This patch equals the behaviour
of initial set and update (using force) in the way, that a missing or undef
value for expiration date sets it to infinity.
The value of 0 is no longer accepted as it had unclear definition.
Adjust datapool calls to changed api
Fix race detaction code in scep server
Fix Mason UI to properly convert datetime info for changed get_cert API