Poll comms: always process standard outputs on poll

[MetRonnie]

In case a task went through >1 state change since the last poll, always process the all outputs gleaned from the job poll info.

However this means that repeated polls will repeat the same outputs, so this PR also needs:

• Prevent task events manager logging the same message more than once #5562

Check List

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to both setup.cfg (and conda-environment.yml if present).
• Tests are included (or explain why tests are not needed).
• Changelog entry included if this is a change that can affect users
• Cylc-Doc pull request opened if required at cylc/cylc-doc/pull/XXXX.
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.