Ignore old messages sent to conductor

Conductor can at present receive updates via heartbeat and update_backup that could potentially arrive out of order, which means an instance's status or that of its backups could be corrupted should a message in the queue fall out of order, especially in the case when a user is employing multiple conductor workers. One proposal to avert this behavior is to attach a "sent" field to each message sent, and discard the ones that arrive to conductor after more recently-sent messages.