Improve transaction test applications to test serializability

Transaction test applications are currently not suitable for testing atomicity/isolation. This can be easily done for strict atomicity/isolation by simply testing that a set of values represent a single transaction. For example, run three concurrent transactions setting three different values on n keys, and run concurrent transactions reading those keys and verifying that all keys have the same value. Retry transactions until they're successful. That ensures that the read transactions are isolated from write transactions and that writes within transactions are atomic.