cache prefixes for prefix-based iteration

postgres/db/migrations/00003_create_eth_key_preimages_table.sql

@@ -1,6 +1,7 @@
 -- +goose Up
 CREATE TABLE IF NOT EXISTS eth.key_preimages (
   eth_key BYTEA UNIQUE NOT NULL,
+  prefix BYTEA,
   ipfs_key TEXT NOT NULL REFERENCES public.blocks (key) ON DELETE CASCADE DEFERRABLE INITIALLY DEFERRED
 );