sbt/sbt#2754 offers a way to cache the classloader that contains the incremental compiler, rather than creating a new one each time. This can deliver significant performance improvements due to the way that JIT works.

• Update to SBT 0.13.13
• Use the new method AnalyzingCompiler#withClassLoaderCache to enable the new classloader recycling.
• Verify that new instances of xsbt.CallbackGlobal are not classloaded in the steady state (java -verbose:class should stop logging.)