Associate receipts with blocks instead of transactions

- Allows us to fetch receipts by block and persist even when
associated transaction is unknown. (The associated transaction
can still be derived from the tx_hash column at query time, but
is an expensive operation to require for inserts).

db/migrations/1525122581_associate_receipts_with_blocks.down.sql

@@ -0,0 +1,23 @@
+BEGIN;
+
+ALTER TABLE receipts
+  ADD COLUMN transaction_id INT;
+
+UPDATE receipts
+  SET transaction_id = (
+    SELECT id FROM transactions WHERE transactions.hash = receipts.tx_hash
+  );
+
+ALTER TABLE receipts
+  ALTER COLUMN transaction_id SET NOT NULL;
+
+ALTER TABLE receipts
+  ADD CONSTRAINT transaction_fk
+FOREIGN KEY (transaction_id)
+REFERENCES transactions (id)
+ON DELETE CASCADE;
+
+ALTER TABLE receipts
+  DROP COLUMN block_id;
+
+COMMIT;

db/migrations/1525122581_associate_receipts_with_blocks.up.sql

@@ -0,0 +1,23 @@
+BEGIN;
+
+ALTER TABLE receipts
+  ADD COLUMN block_id INT;
+
+UPDATE receipts  
+  SET block_id = (
+    SELECT block_id FROM transactions WHERE transactions.id = receipts.transaction_id
+  );
+
+ALTER TABLE receipts
+  ALTER COLUMN block_id SET NOT NULL;
+
+ALTER TABLE receipts
+  ADD CONSTRAINT blocks_fk
+FOREIGN KEY (block_id)
+REFERENCES blocks (id)
+ON DELETE CASCADE;
+
+ALTER TABLE receipts
+  DROP COLUMN transaction_id;
+
+COMMIT;