Move Bleve search index data from singleton to songlist.

[kimtore]

This patch brings a few important changes:

• Library indexing is asynchronous again.
• Overlapping library updates during indexing aborts the old indexing job, and starts a new one.
• Each songlist might potentionally have their own Bleve index.
• More code documentation.

[codecov-io]

Codecov Report

Merging #106 into master will not change coverage. The diff coverage is 0%.

@@           Coverage Diff           @@
##           master     #106   +/-   ##
=======================================
  Coverage   53.35%   53.35%           
=======================================
  Files          51       51           
  Lines        1818     1818           
=======================================
  Hits          970      970           
  Misses        825      825           
  Partials       23       23

Impacted Files	Coverage Δ
commands/isolate.go	28.12% <0%> (ø)	:arrow_up:

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