Upgrade to Scala 2.12.12, 2.13.3

[xerial]

• https://github.com/scala/scala/releases/tag/v2.12.12
• https://github.com/scala/scala/releases/tag/v2.13.3

These Scala versions added -Xasync option, so that scala-async can be efficiently implemented by using the compile-time information. The previous version of scala-async was a macro-based implementation, which was slow and incomplete.

Related:

• scala-async 1.0.0-M1 https://github.com/scala/scala-async/releases/tag/v1.0.0-M1, which requires -Xasync option. 1.0.0 will be released soon

scala-async will support Scala.js too. This would be useful for writing Future-based async code instead of writing chains of Futures (e.g., Conductor's Future-based operation code will benefit from scala-async). I think reading this SIP https://docs.scala-lang.org/sips/async.html is good enough to understand the motivation why scala-async was developed.

[codecov]

Codecov Report

Merging #1199 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1199   +/-   ##
=======================================
  Coverage   83.20%   83.20%           
=======================================
  Files         289      289           
  Lines       11042    11042           
  Branches      732      732           
=======================================
  Hits         9187     9187           
  Misses       1855     1855           

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