Don't discard `last_change` timestamp on `BfChassisManager::PushChassisConfig` (#733)
Don't discard `last_change` timestamp on `BfChassisManager::PushChassisConfig` (#733)

This PR changes the handling of old `last_change` timestamp on `PushChassisConfig`. We no longer discard it, but keep the last known value. This prevents the return of 0 timestamps between a config push and port event.
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