Proposal: use cached features for certain renderer changes

[cmdcolin]

when any of the renderprops changed, say track height, it currently does a full re-requesting of features. this is because ServerSideFeatureRenderer::render() calls adapter.getFeatures for any renderer change

but, this could potentially be avoided.

from discussion with @garrettjstevens

outline from chat:
serversidefeaturerenderer
render() -> getFeatures
ifNothingsChagned->use this.cachedFeatures[blockKey]

bonus needs
a) system that can dispose of those when you dont need it anymore
b) being able to query across blocks

[cmdcolin]

would help with #snpcoverageopti

[cmdcolin]

I tried this out and the results weren't that big of a change. There is still the render delay from the block reaction, and various other unknown things that still get in the way of having e.g. "instant rerender on height change"

[cmdcolin]

test branch which didn't pan out here https://github.com/GMOD/jbrowse-components/tree/cache

will just close for now