DAOS-19374 vos: iterator changes for multi-buckets per object

[NiuYawei]

• Change obj_pin_bkt() to pin all object shared buckets.
• Ensure dkey bucket is pinned when iterating into dkey sub-trees.
• Fix tree handle leak when nested preparation failed.

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 'Eviction thrash prevention'
Status is 'In Progress'
Labels: 'md_on_ssd2'
https://daosio.atlassian.net/browse/DAOS-19374

[daosbuild3]

Test stage Unit Test bdev completed with status UNSTABLE. https://jenkins-3.daos.hpc.amslabs.hpecorp.net/job/daos-stack/job/daos//view/change-requests/job/PR-19048/1/testReport/

[daosbuild3]

Test stage Functional Hardware Medium MD on SSD completed with status FAILURE. https://jenkins-3.daos.hpc.amslabs.hpecorp.net//job/daos-stack/job/daos/view/change-requests/job/PR-19048/1/execution/node/1732/log