[EuiSuperDatePicker] Invoke onRefresh instead of onTimeChanged when clicking “Refresh”

[sorenlouv]

Follow-up to #1577

Since there is now an onRefresh callback, the update button should no longer call onTimeChanged when an actual refresh was performed (which doesn't change the time).
The ui of the button already distinguishes between the two states (update time range vs. manual refresh).
This is also the same callback that will be called, when the automatic refresh interval is fired.

This is important because the consumer might want to have separate logic for when the time range was changed vs. when a refresh was invoked.

To make this backwards compatible this change only take effect for consumers who have implemented the onRefresh callback (searching through Kibana this will only affect APM)

[jasonrhodes]

FYI don't forget the changelog entry :)

[chandlerprall]

Needs a changelog entry, but changes LGTM