Cleanup RequestManager.cancelRequestAndClose

The use of cancelMessageChannel is quite confusing. The message send to the request manager internal channel can probably moved outside the loop to avoid confusion.

Start by locating RequestManager.cancelRequestAndClose and inspect how cancelMessageChannel is used within its loop. Confirm the internal-channel message can be moved outside the loop without changing cancellation behavior, then run the relevant Go tests for the request manager; done means the cleanup is complete and tests pass.

Written by the indexing model from the issue text.