DAOS-17405 cart: Add env var SWIM_SUBGROUP_SIZE (#16228)

[liw]

Make the number of iping targets, called the subgroup size, tunable by new environment variable SWIM_SUBGROUP_SIZE.

Log new SWIM suspicions at INFO level, including their origins. This may help debugging why a DEAD event happens.

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[bot]]

Ticket title is 'Make the subgroup size tunable' Status is 'Awaiting backport' Labels: 'scrubbed_2.6.5' Job should run at elevated priority (1) https://daosio.atlassian.net/browse/DAOS-17405

[daosbuild3]

Test stage Functional Hardware Medium completed with status FAILURE. https://jenkins-3.daos.hpc.amslabs.hpecorp.net//job/daos-stack/job/daos/view/change-requests/job/PR-17215/2/execution/node/515/log