Identity override count retrieval is slow for large numbers of overrides

[khvn26]

For instance, for an environment with ~50k overrides, retrieving features with override counters takes 15 to 80 seconds.

Things to explore:

1. Look into DynamoDB access pattern and ways to optimise it
2. Cache/store identity counters elsewhere

[matthewelwell]

For now, a valid solution is likely to just add a + next to the icon if there are possibly more identity overrides, using the fact that a next page exists in dynamo.