Improve assembly loading time by moving to main thread

[cmdcolin]

This is an alternative to #1406 that performs assembly loading on the main thread

This improves loading time, and makes it so the assembly is waited on before the app is called initialized

[cmdcolin]

Note that the goals of this PR are knock-on effects from wanting to add a test for the location-search box, which means the assembly has to be initialized

Therefore we

• make LGV displaying conditional on the assembly being loaded (see the LGV initialized getter)
• and perform assembly loading in the main thread

If assembly loading is not done in the main thread, the loading time for a basic LGV is much worse because it has to wait on the webworker JS to download, parse, run, etc before anything is displayed

[cmdcolin]

The tests on this branch for me pass locally but do not pass on travis https://travis-ci.com/github/GMOD/jbrowse-components/builds/200457135

[codecov]

Codecov Report

Merging #1438 (79c3d37) into master (a764c01) will decrease coverage by 0.18%.
The diff coverage is 75.43%.

@@            Coverage Diff             @@
##           master    #1438      +/-   ##
==========================================
- Coverage   60.00%   59.81%   -0.19%     
==========================================
  Files         426      425       -1     
  Lines       19395    19004     -391     
  Branches     4581     4448     -133     
==========================================
- Hits        11638    11368     -270     
+ Misses       7465     7346     -119     
+ Partials      292      290       -2     

Impacted Files	Coverage Δ
...kages/core/BaseFeatureWidget/BaseFeatureDetail.tsx	96.66% <ø> (ø)
packages/core/babel.config.js	0.00% <ø> (ø)
packages/core/pluggableElementTypes/ViewType.ts	81.81% <ø> (-12.30%)	⬇️
packages/core/pluggableElementTypes/WidgetType.ts	81.81% <ø> (-6.42%)	⬇️
...pes/renderers/ComparativeServerSideRendererType.ts	86.00% <ø> (-1.84%)	⬇️
packages/core/rpc/coreRpcMethods.ts	84.31% <ø> (-1.20%)	⬇️
packages/core/ui/App.js	63.63% <ø> (ø)
packages/core/ui/StartScreen.tsx	50.00% <ø> (ø)
packages/core/util/types/index.ts	50.00% <ø> (ø)
packages/development-tools/src/build.ts	94.73% <ø> (ø)
... and 54 more

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[cmdcolin]

This is now tests passing

[rbuels]

if CoreGetRegions and CoreGetRefNameAliases are no longer used, let's add deleting them to this PR

[cmdcolin]

I was somewhat comfortable with this

However, running git checkout origin/master test_data....This reverts the test_data/volvox/config.json back to volvox.2bit and then the tests fail...the volvox.2bit fails while the indexedfasta succeeds. This is somewhat concerning and may be worth looking into

[cmdcolin]

Traced the aforementioned loading failure of volvox.2bit to usage of jest.useFakeTimers being enabled (and then deep in http-range-fetcher depending on usage of a setTimeout). I disabled usage of useFakeTimers for now to fix