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GH-34391: [C++] Future as-of-join-node hangs on distant times (#34392)

See #34391
* Closes: #34391

Note that the `TestBasic7Forward` test-case included in the PR reproduces the hang in the pre-PR code.

Lead-authored-by: Yaron Gvili <rtpsw@hotmail.com>
Co-authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>