If time_expire=0, then the cache should be cleared and reset with the
latest value to stay compatible with documented web2py functionality of
how caching works. Previous change broke this compatibility while adding
efficiency in other cases.
When gae_memcache passed expiration time to memcache Client
(see commit 59290534bc),
logic is no longer needed to calculate time delta, since memcache
handles that for us. Also, time_expire=0 was expiring values
immediately, instead of the documented behavior where 0 signified
no expiration.
Kept timestamp in value for backwards compatibility (is this
necessary since cached data is transient?)
