Disallow transaction to return a Promise due to unexpected behaviour

Co-authored-by: phiresky <[email protected]>

lib/methods/transaction.js

@@ -63,6 +63,9 @@ const wrapTransaction = (apply, fn, db, { begin, commit, rollback, savepoint, re
 	before.run();
 	try {
 		const result = apply.call(fn, this, arguments);
+		if (result && typeof result.then === 'function') {
+			throw new TypeError('Transaction function cannot return a promise');
+		}
 		after.run();
 		return result;
 	} catch (ex) {