qmanager: annotate each job only once on reservation

[trws]

problem: an annotation is sent for each job that enters reserved state when its predicted start time changes. That is controlled by when the old_at and at times associated with the job in qmanager are different. The implementation fails to update old_at after this to prevent repeat messages. This was somewhat hidden by the fact that the subsequent updates had little effect, except after recent performance improvments made it possible for the qmanager to flood the job-manager with annotations in some cases.

solution: set old_at to at after deciding whether to send the annotation

fixes #1249

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 74.4%. Comparing base (f37977b) to head (69a3c71). Report is 133 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1250 +/- ## ====================================== Coverage 74.4% 74.4% ====================================== Files 104 104 Lines 14936 14937 +1 ====================================== + Hits 11118 11119 +1 Misses 3818 3818 ``` | [Files with missing lines](https://app.codecov.io/gh/flux-framework/flux-sched/pull/1250?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=flux-framework) | Coverage Δ | | |---|---|---| | [qmanager/modules/qmanager\_callbacks.cpp](https://app.codecov.io/gh/flux-framework/flux-sched/pull/1250?src=pr&el=tree&filepath=qmanager%2Fmodules%2Fqmanager_callbacks.cpp&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=flux-framework#diff-cW1hbmFnZXIvbW9kdWxlcy9xbWFuYWdlcl9jYWxsYmFja3MuY3Bw) | `72.4% <100.0%> (+0.1%)` | :arrow_up: |