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 invalid values for the 'fixed_cost' field of the 'pool_update'
table being display as negative value.

The constraint will be droppped only if it exists (eg someone had previously
dropped in manually).

Author: erikd

schema/migration-1-0004-20201020.sql

@@ -0,0 +1,11 @@
+CREATE FUNCTION migrate() RETURNS void AS $$
+
+BEGIN
+  EXECUTE 'ALTER DOMAIN lovelace DROP CONSTRAINT IF EXISTS lovelace_check; ';
+END;
+
+$$ LANGUAGE plpgsql;
+
+SELECT migrate();
+
+DROP FUNCTION migrate();