Add abortable promise cache for recovering from index loading error

[cmdcolin]

The code in the last PR #73 went some ways towards enabling reload, however it was not able to recover from a 404 on the index file

This makes it so that the index is fetched from the abortable-promise-cache, basically an abortable-memoize. Goes some ways towards #27 and IMO is fairly easy to reason about

Con: added dependencies, but hopefully not too bad

Goes to the larger goal of enabling better recovery from errors

[codecov]

Codecov Report

Merging #74 into master will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #74      +/-   ##
==========================================
+ Coverage   89.68%   89.70%   +0.02%     
==========================================
  Files          34       34              
  Lines        1512     1515       +3     
==========================================
+ Hits         1356     1359       +3     
  Misses        156      156              

Impacted Files	Coverage Δ
src/craiIndex.js	96.42% <100.00%> (+0.13%)	⬆️

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[cmdcolin]

In case it helps, the test from #73 only performed a test for the CRAM file itself recovering, so this tests for the CRAI in this case (didn't try other index formats)

[cmdcolin]

There is a test for this behavior now that fails on master but succeeds here