Object timestamp does not match log timestamp

[songweijia]

In PersistentCascadeStore<>, ObjectType::ver is a tuple with a version number and timestamp. In the current implementation, the timestamp is set in PersistentCascadeStore<>::ordered_put(). However, in derecho::Persistent<>::set(), the timestamp is assigned on message delivery and passed through persistent_manager. That timestamp will becoming the timestamp index in the log entry. Therefore, the timestamp in ObjectType is slighly earlier than the log entry timestamp.

To fix that issue, we should allow Persistent<>::set() to update the timestamp in ObjectType on delivery.