Move DictKeyValueStore from test.tools to xblock.runtime
Rename DbModel to KvsFieldData.
Make KvsFieldData work properly in multiple-inheritance uses.
Parents and children can't possibly have defaults

Don't bother looking for defaults for "parents" and "children".
Add a caching .get_parent() method to XBlock.
Move MemoryUsageStore to xblock.runtime
Implement get_block uniformly on core Runtime.

Once we have usage_store as a member of Runtime, we should use it to
implement get_block.
MemoryUsageStore now makes slightly nicer ids.
Use .user_id in WorkbenchRuntime.