Ensure single task reads single partition in write stage

[losipiuk]

Documentation

(x) No documentation is needed.
( ) Sufficient documentation is included in this PR.
( ) Documentation PR is available with #prnumber.
( ) Documentation issue #issuenumber is filed, and can be handled later.

The thing which could be documented is config/sessino property, yet I think it is not needed.
I added it merely as a kill switch, and changing default should not be needed.

Release notes

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# General
* Increase high granularity of tasks for stages which are writing table data, when fault-tolerant execution is enabled (`retry-policy` is set to `TASK`).
  That is to reduce chance of hitting `Exceeded limit of N open writers for partitions` errors. ({issue}`12721`)

[linzebing]

Shall we do:

(preserveInputPartitionsInWriteStage && isWriteFragment(fragment))

So we can shortcut the logic and avoid traversing the plan tree is flag is not set?

[losipiuk]

Yeah - makes sense