[C++][Compute] Provide a default implementation of ExecNode::Pause/Resume

[asfimport]

ExecNode::PauseProducing, ResumeProducing are currently stubs, but even after backpressure is implemented many nodes will simply forward backpressure to their own inputs. Providing a default implementation of these methods will alleviate boilerplate and simplify implementation of backpressure

Reporter: Ben Kietzman / @bkietz

_{Note: This issue was originally created as ARROW-13659. Please see the migration documentation for further details.}

[asfimport]

Weston Pace / @westonpace:
How confident are we that backpressure will be applied in this way? I've been wondering if we need PauseProducing/ResumeProducing at all. An alternative implementation is to have special data holder nodes which are integrated with a scheduler. Only the data holder nodes queue data and have to worry about sending back pressure signals to the scheduler. In this model there is no need for every node to send back pressure signals (since most nodes can't do anything about them anyways, as you point out).

[github-actions]

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