feat: add updated_at postgres fn + trigger

* closes #66

migrations/003_gas_quota.sql

@@ -25,4 +25,4 @@ $$ language plpgsql;
 create trigger insert_gas_quota
     after insert on keystore
 for each row
-execute procedure insert_gas_quota()
+execute procedure insert_gas_quota();

migrations/004_updated_at.sql

@@ -0,0 +1,33 @@
+ALTER TABLE keystore
+ADD COLUMN updated_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP;
+
+ALTER TABLE otx_dispatch
+ADD COLUMN updated_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP;
+
+ALTER TABLE gas_quota
+ADD COLUMN updated_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP;
+
+-- updated_at function
+create function update_timestamp()
+    returns trigger
+as $$
+begin
+    new.updated_at = current_timestamp;
+    return new;
+end;
+$$ language plpgsql;
+
+create trigger update_keystore_timestamp
+    before update on keystore
+for each row
+execute procedure update_timestamp();
+
+create trigger update_otx_dispatch_timestamp
+    before update on otx_dispatch
+for each row
+execute procedure update_timestamp();
+
+create trigger update_gas_quota_timestamp
+    before update on gas_quota
+for each row
+execute procedure update_timestamp();