stocon: bump number of concurrent reconciles per operation

[VladLazar]

Problem

Background node operations take a long time for loaded nodes.

Summary of changes

Increase number of concurrent reconciles an operation is allowed to spawn.
This should make drain and fill operations faster and the new value is still well
below the total limit of concurrent reconciles.

Checklist before requesting a review

• I have performed a self-review of my code.
• If it is a core feature, I have added thorough tests.
• Do we need to implement analytics? if so did you add the relevant metrics to the dashboard?
• If this PR requires public announcement, mark it with /release-notes label and add several sentences in this section.

Checklist before merging

• Do not forget to reformat commit message to not include the above checklist

[github-actions]

2965 tests run: 2849 passed, 0 failed, 116 skipped (full report)

---

Flaky tests (3)

Postgres 15

• test_pageserver_chaos[None]: release

Postgres 14

• test_slow_secondary_downloads[False]: release
• test_replication_start: release

Code coverage* (full report)

• functions: 32.7% (6896 of 21118 functions)
• lines: 50.0% (53926 of 107861 lines)

* collected from Rust tests only

---

_{The comment gets automatically updated with the latest test results
2ee37d4 at 2024-06-27T13:25:08.026Z :recycle:}