sql: use soft limits for scans

[yuzefovich]

In 20.1 release we extended the propagation of the soft limits, so now we have them on scan nodes. However, currently we're ignoring soft limits since the TableReaders will still read too eagerly in the soft limit case. To prevent this we'll need a new mechanism on the execution side to modulate table reads.

Jira issue: CRDB-4420

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[yuzefovich]

this effectively has been addressed by #160051