Make memcache use default namespace.

[jongillham]

This fixes bug #42 by ensuring all memcached entities use the default namespace despite the namespace of their respective keys.

[luna-duclos]

Wouldn't this cause weird issues if a two entities in different namespaces have the same key ? I believe the namespace chosen for memcache should always be the one from the key

[jongillham]

@PSG-Luna that was my original though. Unfortunately it is not scalable to have memcach.Items use the same namespace as the datastore.Keys associated with them. This is because memcache.*Multi will not allow putting multiple entities into different namespaces in one call, unlike datastore.*Multi.

The way around it is to put all memcached entities into one namespace (currently the default) and do 'namespacing' ourselves. If you look at createMemcacheKey(key *datastore.Key) you will see it uses datastore.Key.Encode() which encodes the namespace as well. This means that there will be no key collisions between identical keys in different namespaces.

What do you think?

[luna-duclos]

Seems reasonable enough, thanks :)