Support cached load in batch in CachedDatastore about appx-old HOT 1 CLOSED

appx.CachedDatastore#LoadAll tries to:

1. In case none of the entities are cached, It loads all entities in a single batch from datastore.
2. Else, it loads all the given entities from the cache in a single batch
3. For each cache miss it encounters, it then falls back to appx.CachedDatastore#Load which then does:
   1. In case the entity does not implement appx.CacheMissQueryable, appx will attempt to reconstruct the entity's key based on the entity's implementation of appx.Persistable#KeyMetadata so it can be looked up from datastore
   2. In case the entity implements appx.CacheMissQueryable and appx is not able to reconstruct its key then the entity's data is retrieved from datastore by running the provided cache miss query.
4. Else, an error is returned

Still need to think of the best way to handle errors. Perhaps LoadAll is able to partially load the give slice in case an entity is not in the cache nor in datastore, should an ErrPartiallyLoaded be returned at the end of the process or should the load process rollback any changes to the slice?

from appx-old.