Only enable optimizer in CI

[mdedetrich]

From https://docs.scala-lang.org/overviews/compiler-options/optimizer.html#in-brief

Don’t enable the optimizer during development: it breaks incremental compilation, and it makes the compiler slower. Only enable it for testing, on CI, and to build releases.

This PR makes it so that the Scala 2 optimizer is only enabled in CI since it causes issues when used in local development with additional logging to make it clear when its enabled.

Note That there is a presumption in that Scala modules that use this sbt-plugin do their releases in CI rather than a local machine. @SethTisue would be good to check if this is the case, if not then then let me know which Scala modules don't use CI for release and I can look into enabling it (I do think that by now all Scala modules should be creating release artifacts in CI).

[SethTisue]

Note That there is a presumption in that Scala modules that use this sbt-plugin do their releases in CI rather than a local machine. @SethTisue would be good to check if this is the case

Confirmed, this is the case.

Let's find out what Mr. Optimizer himself, @lrytz, thinks about this PR

[SethTisue]

@lrytz ping :-)

[lrytz]

LGTM, missed it... thanks for the ping.

[SethTisue]

Note That there is a presumption in that Scala modules that use this sbt-plugin do their releases in CI rather than a local machine

I've added a commit that adds a statement to this effect to the Scaladoc for the setting.

And yes, all the modules are currently released from CI using sbt-ci-release.

(And I think only scala-async actually enables the optimizer.)