related to issue 1708 - bugfix to allow no expiration for gae_memcache

When gae_memcache passed expiration time to memcache Client
(see commit 59290534bc2faad501ba13c495aeb051d619314),
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?)