Memory optimisations for the local asset manager.

Summary:
This optimizes the local asset manager by preventing storage of raw assets.  Raw assets are stored and loaded from disk, and not stored in memory beyond the initial deserialisation.

This change brings the memory usage of Tychaia down from 1.7GB to 900MB after the game starts.

Test Plan: N/A, for review only.

Reviewers: Sabre070

Reviewed By: Sabre070

Maniphest Tasks: T112

Differential Revision: http://code.redpointsoftware.com.au/D95