Implement asset metadata storage in Split - per-asset functionality
PLAT-40

Conflicts:
	common/lib/xmodule/xmodule/assetstore/__init__.py
	common/lib/xmodule/xmodule/modulestore/__init__.py
	common/lib/xmodule/xmodule/modulestore/mixed.py
	common/lib/xmodule/xmodule/modulestore/mongo/base.py
Moved assetstore tests to own file and ddt'd across stores

Conflicts:
	common/lib/xmodule/xmodule/modulestore/tests/test_mongo.py
Merge pull request #5723 from edx/dhm/split_assetstore

Implement asset metadata storage in Split - per-asset functionality