dorb

Known issues

When a messageset is produced with the RequiredAcks set at 0, the waiting client will time out (if called with send_sync). This will also leak #waiter{} records in dorb_socket;
CRC32 validation is not done on incoming messages;
If a dorb_socket process crashes, it will leave callers waiting until they time out. (Fix is to monitor the dorb_socket process and handle the DOWN message gracefully - monitoring also returns a ref that could be reused).