Revert default search limit to 100 and make it configurable

[ddelemeny]

The possible pagination issues mentioned in #97 should be thoroughly tested.

[ddelemeny]

Quick and dirty solution would be to create a new datasource parameter to be changed in the config editor, but that would pollute the options with UI defaults (which are persisted in panel defs and whatnot), it's not made for that.

Better approach would be to create a config store for persistent UI settings, it's more involved but won't bloat the datasource definition.

[ddelemeny]

Defaults are currently static, implemented deep within the QueryEditor.
src/components/QueryEditor/MetricAggregationEditor/utils.ts:metricAggregationConfig

This needs to be refactored.