Last ndt7 ServerMeasurement snapshot not taken at end of test

[stephen-soltesz]

The ndt7 archival format does not include the average rate observed by the client or server. So, the data pipeline uses the last observed tcpinfo snapshot to calculate the average rates. https://github.com/m-lab/etl/blob/main/parser/ndt7_result.go#L165-L181

However, the ndt7 server does not guarantee when these snapshots are observed and recorded in the ServerMeasurements. This means that the archived last snapshot may not occur near the actual close of the connection. https://github.com/m-lab/ndt-server/blob/main/ndt7/download/sender/sender.go#L72-L86

Ideally, when the server is shutting down the connection it would take and record one final snapshot.