(VDB-354) Queue unrecognized storage diffs

- If we recognize a storage diff as coming from a watched contract but
  don't recognize the key, queue it for retrying later (after we've seen
  an event that might help us recognize the key)
- Remove unused errs and args
- Panic on unrecognized types (should not happen)

db/migrations/20190219134901_create_queued_storage.sql

@@ -0,0 +1,12 @@
+-- +goose Up
+CREATE TABLE public.queued_storage (
+  id            SERIAL PRIMARY KEY,
+  block_height  BIGINT,
+  block_hash    BYTEA,
+  contract      BYTEA,
+  storage_key   BYTEA,
+  storage_value BYTEA
+);
+
+-- +goose Down
+DROP TABLE public.queued_storage;