[Metrics App] Waffle map auto-refresh loading state is too disruptive

[jasonrhodes]

When a user sets the waffle map to "Auto Refresh", each load redraws the entire map (and hides the existing map while loading is in progress). See the below image, especially as I try to interact with the hover-state tooltips and click a link to investigate the metrics/logs/etc of a specific node.

AC:

• Loading state should not interrupt or remove the existing, already loaded data until after the new data has been loaded
• Loading must still be represented somehow in the UI so that the user knows that loading is happening in the background
• Can we maintain a link to individual nodes (via React keys) so that only different nodes are actually re-drawn/updated?

[elasticmachine]

Pinging @elastic/logs-metrics-ui (Team:logs-metrics-ui)