[Backport 5.2] repair: Introduce repair_partition_estimation_ratio config option

[mergify]

In commit 642f9a1 (repair: Improve estimated_partitions to reduce memory usage), a 10% hard coded estimation ratio is used.

This patch introduces a new config option to specify the estimation ratio of partitions written by repair out of the total partitions.

It is set to 0.1 by default.

Fixes #18615

(cherry picked from commit 340eae0)

Refs #18634

[mergify]

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Changes to be committed:
	modified:   repair/row_level.cc

Unmerged paths:
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	both modified:   db/config.cc
	both modified:   db/config.hh

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