Updates, refactorings, generic tests in monifu-core

- update Scala/Scala.js version
- migrate JVM tests to MiniTest in the shared folder
- add TestScheduler as a shared Scheduler that can
  be used in shared tests
- add missing java.util.concurrent.TimeoutException
  in the JS version
- add AtomicInt, AtomicLong, AtomicFloat, AtomicDouble
  in the JS version
- refactor monifu.concurrent.atomic.padded in the
  JS version