reduce LGV block rerenders to speed up scrolling

[rbuels]

Right now, the Block and Ruler elements updating as you scroll is starting to have a real performance impact.

It's due to code like this:

https://github.com/GMOD/jbrowse-components/blob/master/packages/linear-genome-view/src/LinearGenomeView/components/ScaleBar.js#L59

and this:

https://github.com/GMOD/jbrowse-components/blob/master/packages/linear-genome-view/src/BasicTrack/components/TrackBlocks.js#L56

Need to reduce the rerenders of these blocks, using the techniques in https://mobx.js.org/best/react-performance.html

[rbuels]

A good 50% of the scrolling frame runtime is taken up by these block updates, which could probably be mostly eliminated:

[cmdcolin]

Might be related to facebook/react#8669 since Block uses {props.children}

[cmdcolin]

This is done to some extent. Further profiling could be done