Batch writes of instance exit codes

[shamsimam]

Is your feature request related to a problem? Please describe.

This line in scheduler.c;lj seems problematic:
(datomic/transact-with-retries conn [[:db/add instance-id :instance/exit-code (int exit-code)]])))))

Reduces the overhead of writing to datomic via batching.

Describe the solution you'd like

Use a process similar to the progress messages or sandbox syncer to aggregate and reduce number of transactions sent to datomic.

Describe alternatives you've considered

None.

Additional context

We are wary of the load we are putting on datomic. There is a feeling that aggregating transactions mitigates against the scenario of overloading datomic.