Move storage classes into a main "data store".

This is in preparation for having multiple data stores that offer
different functionality, e.g. splitting out state or event storage.

synapse/storage/schema/full_schemas/54/full.sql

@@ -0,0 +1,8 @@
+
+
+CREATE TABLE background_updates (
+    update_name text NOT NULL,
+    progress_json text NOT NULL,
+    depends_on text,
+    CONSTRAINT background_updates_uniqueness UNIQUE (update_name)
+);