Backfill Frob log events

db/migrations/1533844125_create_frob_table.down.sql

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+DROP TABLE maker.frob;

db/migrations/1533844125_create_frob_table.up.sql

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+CREATE TABLE maker.frob (
+  id        SERIAL PRIMARY KEY,
+  header_id INTEGER NOT NULL REFERENCES headers (id) ON DELETE CASCADE,
+  tx_idx    INTEGER,
+  ilk       bytea,
+  lad       bytea,
+  gem       NUMERIC,
+  ink       NUMERIC,
+  art       NUMERIC,
+  era       NUMERIC,
+  UNIQUE (header_id, tx_idx)
+);