Add constraint to prevent duplicate headers

- Disallow inserts of headers with the same number, hash, and node
  fingerprint, since it will enable duplicate log fetching for the
  same header

db/migrations/00023_create_headers_table.sql

@@ -8,7 +8,8 @@ CREATE TABLE public.headers
     block_timestamp      NUMERIC,
     check_count          INTEGER NOT NULL DEFAULT 0,
     eth_node_id          INTEGER NOT NULL REFERENCES eth_nodes (id) ON DELETE CASCADE,
-    eth_node_fingerprint VARCHAR(128)
+    eth_node_fingerprint VARCHAR(128),
+    UNIQUE (block_number, hash, eth_node_fingerprint)
 );
 
 -- Index is removed when table is