txnsync: fix potential race during TestBasicCatchpointCatchup #3033
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Summary
During fast catchup, we restart the transaction sync service very quickly.
This can cause a network message being sent, and the response would be returned to the "restarted" txnsync.
Since we don't want to disconnect the network connection itself ( which could have some messages enqueued ), the transaction sync would need to store the "returned" channel before sending the message. That would avoid the data race ( and safely ignore the incoming message ).
Test Plan
Use existing testing, and confirm against that.