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/schema.sql

@@ -2,8 +2,8 @@
 -- PostgreSQL database dump
 --
 
--- Dumped from database version 11.2
--- Dumped by pg_dump version 11.2
+-- Dumped from database version 11.3
+-- Dumped by pg_dump version 11.3
 
 SET statement_timeout = 0;
 SET lock_timeout = 0;
@@ -12,6 +12,7 @@ SET client_encoding = 'UTF8';
 SET standard_conforming_strings = on;
 SELECT pg_catalog.set_config('search_path', '', false);
 SET check_function_bodies = false;
+SET xmloption = content;
 SET client_min_messages = warning;
 SET row_security = off;
 
@@ -789,6 +790,14 @@ ALTER TABLE ONLY public.eth_nodes
     ADD CONSTRAINT nodes_pkey PRIMARY KEY (id);
 
 
+--
+-- Name: queued_storage queued_storage_block_height_block_hash_contract_storage_key_key; Type: CONSTRAINT; Schema: public; Owner: -
+--
+
+ALTER TABLE ONLY public.queued_storage
+    ADD CONSTRAINT queued_storage_block_height_block_hash_contract_storage_key_key UNIQUE (block_height, block_hash, contract, storage_key, storage_value);
+
+
 --
 -- Name: queued_storage queued_storage_pkey; Type: CONSTRAINT; Schema: public; Owner: -
 --