[Dvaas] Rename time_limit to packet_synthesis_time_limit. Create function to access packet_synthesis_timed_out in ValidationResult. Add more comments to PacketSynthesizerTimedOut.Add support for non-entry entities.

[Dvaas] Rename time_limit to packet_synthesis_time_limit. Create function to access packet_synthesis_timed_out in ValidationResult. Add more comments to PacketSynthesizerTimedOut.Add support for non-entry entities. #3861