db: Drop constraint on lovelace database type

This is a workaround fix for:

   #351

This fix will not require the database to be re-synced, but will result
in some bad values being display as negative value.

schema/migration-1-0004-20201020.sql

@@ -0,0 +1,41 @@
+-- Hand written migration to create the custom types with 'DOMAIN' statements.
+
+CREATE FUNCTION migrate() RETURNS void AS $$
+
+DECLARE
+  next_version int;
+
+BEGIN
+  SELECT stage_one + 1 INTO next_version FROM "schema_version";
+  IF next_version = 1 THEN
+    EXECUTE 'ALTER DOMAIN lovelace DROP CONSTRAINT lovelace_check; ';
+    EXECUTE 'CREATE DOMAIN txindex AS smallint CHECK (VALUE >= 0 AND VALUE < 1024);';
+    EXECUTE 'CREATE DOMAIN uinteger AS integer CHECK (VALUE >= 0);';
+
+    -- Blocks, transactions and merkel roots use a 32 byte hash.
+    EXECUTE 'CREATE DOMAIN hash32type AS bytea CHECK (octet_length (VALUE) = 32);';
+
+    -- Addresses use a 28 byte hash (as do StakeholdIds).
+    EXECUTE 'CREATE DOMAIN hash28type AS bytea CHECK (octet_length (VALUE) = 28);';
+
+    -- Stake addresses are a 28 byte hash prepended with a byte describing the address.
+    EXECUTE 'CREATE DOMAIN addr29type AS bytea CHECK (octet_length (VALUE) = 29);';
+
+    -- 'maxBound :: Word128' as a decimal has 39 digits, so we only need to check that it
+    -- is positive.
+    EXECUTE 'CREATE DOMAIN word128type AS numeric (38, 0) CHECK (VALUE >= 0);';
+
+    -- 'maxBound :: Word64' as a decimal has 20 digits but not all 20 digit values are less than
+    -- 'maxBound :: Word64'.
+    EXECUTE 'CREATE DOMAIN word64type AS numeric (20, 0) CHECK (VALUE >= 0 AND VALUE <= 18446744073709551615);';
+
+    UPDATE "schema_version" SET stage_one = 1;
+    RAISE NOTICE 'DB has been migrated to stage_one version %', next_version;
+  END IF;
+END;
+
+$$ LANGUAGE plpgsql;
+
+SELECT migrate();
+
+DROP FUNCTION migrate();