Don't duplicate queued storage diffs

- currently, if we don't recognize the same diff several times (e.g.
  if you restart the storage diff watcher pointed at the same file),
  we'll add the same row to the queue on each run.
- these changes assure we only queue an unrecognized diff once.

db/migrations/00024_create_queued_storage.sql

@@ -5,7 +5,8 @@ CREATE TABLE public.queued_storage (
   block_hash    BYTEA,
   contract      BYTEA,
   storage_key   BYTEA,
-  storage_value BYTEA
+  storage_value BYTEA,
+  UNIQUE (block_height, block_hash, contract, storage_key, storage_value)
 );
 
 -- +goose Down