tor-proto: Rename misleading field in `DataCmdChecker`.

gabi-250requested to merge
gabi-250/arti:tor-proto-todos into main

This is a follow-up from !1451 (merged).

This commit solves a TODO HSS introduced when DataCmdChecker got an additional constructor (new_connected) for creating "pre-connected" streams. See f6745d31 for more details.

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