Initialize Enso Terminal in a background thread

[JaroslavTulach]

Pull Request Description

Fixes #8688 by starting a background thread to initialize whole JLine infrastructure when executed with --repl option..

Checklist

Please ensure that the following checklist has been satisfied before submitting the PR:

• All code follows the
  Java,
• Unit tests continue to pass
• Startup benchmarks aren't worse (it is unlikely they can get better - since Must specify --repl to enable debug server #10709 we no longer initialize JLine when not started with --repl option): initial run

[JaroslavTulach]

None of the startup scenarios is affected:

All are in the expected delta. Expected - we don't measure startup with --repl and that's the only one affected by initializing Enso terminal in a background.