Lazy-load assemblies on demand instead of all at app startup

[cmdcolin]

The current code for jbrowse-web/jbrowse-desktop loads ALL configured assemblies and their refnames on startup e.g. fetches their "fai" file or similar on startup. While these fai files are not often too large (unless assembly is fragmented, then can be megabytes, or if it is twobit, which has very poor access pattern with a range request for each refname in the file), but even for small ones, it is a bit of a network stampede. A user requested loading 1000 assemblies in gitter. This PR helps by lazily loading assemblies as the data is requested, either via assemblyManager.get('asm') or assemblyManager.waitForAssembly('asm')

The get() is a little funky since it is a "view" that kicks off an async action to start the loading even though it returns synchronously, but code that uses assemblyManager.get should know about this type of reactivity

One change for this is that LGV.navToLocString is now async. This is needed to handle locstrings that contain an assembly name which we test for, e.g. {volvox2}:ctgA:1-100

Some tests are updated accordingly to not immediately check the result of navToLocString

Fix #1735

[codecov]

Codecov Report

Merging #3287 (2e3bc07) into main (fd08109) will increase coverage by 0.29%.
The diff coverage is 80.92%.

@@            Coverage Diff             @@
##             main    #3287      +/-   ##
==========================================
+ Coverage   59.40%   59.70%   +0.29%     
==========================================
  Files         679      675       -4     
  Lines       28868    28735     -133     
  Branches     7047     6981      -66     
==========================================
+ Hits        17150    17157       +7     
+ Misses      11441    11334     -107     
+ Partials      277      244      -33     

Impacted Files	Coverage Δ
packages/core/rpc/WebWorkerRpcDriver.ts	0.00% <0.00%> (ø)
products/jbrowse-img/src/renderRegion.js	0.00% <0.00%> (ø)
...se-react-linear-genome-view/src/createViewState.ts	54.16% <10.00%> (+2.31%)	⬆️
packages/core/assemblyManager/assemblyManager.ts	72.30% <66.66%> (+0.51%)	⬆️
.../linear-genome-view/src/LinearGenomeView/index.tsx	83.42% <70.83%> (+0.25%)	⬆️
...-react-circular-genome-view/src/createViewState.ts	86.36% <87.50%> (-4.12%)	⬇️
packages/core/rpc/coreRpcMethods.ts	83.33% <95.23%> (+2.88%)	⬆️
packages/core/assemblyManager/assembly.ts	90.50% <97.87%> (+0.37%)	⬆️
packages/core/rpc/MainThreadRpcDriver.ts	66.66% <100.00%> (ø)
plugins/sequence/src/IndexedFastaAdapter/index.ts	75.00% <0.00%> (-25.00%)	⬇️
... and 19 more

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[cmdcolin]

managed to pass all tests now, should be ready to go :)