storcon: deny external node configuration if an operation is ongoing

[koivunej]

Per #8674, disallow node configuration while drain/fill are ongoing. Implement it by adding a only-http wrapper Service::external_node_configure which checks for operation existing before configuring.

Additionally:

• allow cancelling drain/fill after a pageserver has restarted and transitioned to WarmingUp

Fixes: #8674

[github-actions]

2164 tests run: 2095 passed, 0 failed, 69 skipped (full report)

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Flaky tests (1)

Postgres 15

• test_delete_timeline_client_hangup: release

Code coverage* (full report)

• functions: 32.4% (7219 of 22266 functions)
• lines: 50.4% (58287 of 115727 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
0ee542f at 2024-08-15T09:59:50.707Z :recycle:}