Possible stale cache when using Delete and a memcache lock fails.

[jongillham]

nds.Delete currently locks memcache and then deletes its entity from the datastore. If the following situation occurs then stale cache will be left in the datastore:

• Delete is called and locks the memcache item to be deleted.
• The memcache lock is evicted from memcache.
• Get is called and loads the entity into memcache.
• Delete removes the entity from the datastore.

Therefore the entity will no longer be in the datastore but will be in memcache.

Note that this issue does not affect consistency guarantees when just using nds.Get* and nds.Put* functions.