DAOS-19202 client: Fix cont_create use-after-frees (#18515)

[liw]

Calling daos_cont_create_with_label may result in use-after-frees when underlying tasks reinitialize:

cont_prop  ev        Use-after-free
----------------------------------------------------------------
NULL       non-NULL  merged_props in daos_cont_create
RF         non-NULL  merged_props in daos_cont_create_with_label

This patch refactors daos_cont_create_with_label and daos_cont_create to free properties they allocate in task completion callbacks.

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 'Use-after-free issues in daos_cont_create_with_label when events are used'
Status is 'In Progress'
Labels: 'request_for_2.8'
https://daosio.atlassian.net/browse/DAOS-19202