DAOS-18368 rebuild: fix use before check of ec_agg_boundary

[liuxuezhao]

1. fix a bug of using ec_agg_boundary before checking its valid
2. add some more logs for rebuild fetch getting zero iod_size,
   to provide some hints for layout information.
3. add some DEBUG logs for rebuild enumerate
4. add WARN log if some engines did not report EC agg epoch progress in
   600S.

 Signed-off-by: Xuezhao Liu <xuezhao.liu@hpe.com>

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 'Data corruption observed with master branch under MDonSSD environment.'
Status is 'In Progress'
Labels: 'md_on_ssd,scrubbed_2.8'
https://daosio.atlassian.net/browse/DAOS-18368

[daosbuild3]

Test stage NLT on EL 8.8 completed with status FAILURE. https://jenkins-3.daos.hpc.amslabs.hpecorp.net/job/daos-stack/job/daos/job/PR-17324/2/display/redirect

[daosbuild3]

Test stage NLT on EL 8.8 completed with status UNSTABLE. https://jenkins-3.daos.hpc.amslabs.hpecorp.net/job/daos-stack/job/daos//view/change-requests/job/PR-17324/4/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-17324/4/execution/node/1313/log

[daosbuild3]

Test stage Functional Hardware Medium Verbs Provider 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-17324/4/execution/node/1323/log