Let memcache store explicitly serialise / deserialize data

In MDL-46151, we discovered that some inconsistencies in the PHP memcache module are causing cached data to be returned as a different type from what was set.

To help prevent problems, it would be very useful if the memcache store were to have a setting to allow the user to select whether to explicitly serialise data as part of the set() operation, and to deserialise it in the get() operation before returning it to the calling code.