Remove KeyValueStore.default
Clean up the Workbench KVS somewhat

This KeyValueStore is not the MemoryKeyValueStore, it's particular to
the workbench, so name it appropriately, and make clear what is
Workbench, and what is KVS.
Move DictKeyValueStore from test.tools to xblock.runtime
Rename DbModel to KvsFieldData.
Provide more information in some exceptions.