DAOS-18618 pool: fix pool destroy hang during stop (#18284)

[wangshilong]

Pool destroy can hang with an active scrubber because two scrubber cleanup bugs combine during teardown:

1. cont_iter_is_loaded_cb() can exit without calling sc_cont_teardown(), leaving sc_scrubbing set and blocking destroy on sc_scrub_cond.
2. sc_ensure_containers_are_loaded() can loop forever on DER_CONT_NONEXIST after the container has already been removed.

Fix the teardown path so scrubber state is always released, skip removed containers, and stop scrubber work promptly when the pool is stopping.

Steps for the author:

• Commit message follows the guidelines.
• Appropriate Features or Test-tag pragmas were used.
• Appropriate Functional Test Stages were run.
• At least two positive code reviews including at least one code owner from each category referenced in the PR.
• Testing is complete. If necessary, forced-landing label added and a reason added in a comment.

After all prior steps are complete:

• Gatekeeper requested (daos-gatekeeper added as a reviewer).

[github-actions]

Ticket title is 'daos_test/suite.py:DaosCoreTest.test_daos_rebuild_simple - test timeout w/ DER_CSUM err'
Status is 'Reopened'
Labels: '2.8.0tb5,ci_2.6_daily,ci_2.8_daily,ci_master_daily,pr_test,scrubbed_2.8,test_2.8'
Job should run at elevated priority (1)
https://daosio.atlassian.net/browse/DAOS-18618