Merge branch stable-2.15 into master

* origin/stable-2.15:
  Use java.util.concurrent.ScheduledThreadPoolExecutor for tests
  Document that cache cleaning intervals < 1h are not supported
  Apply changes to file-based websession cache from HA-plugin
  Update cache cleaner to HA-plugin version and add tests
  Add additional tests for websession cash from HA-plugin
  Refactor test key assignment in cache tests
  Use junit to provide a temporary directory for testing
  Simplify cache-directory creation
  Adapt system time manipulation to method used in HA-plugin
  Reformat code with google-java-format 1.7
  Fix getIfPresentObjectNonStringTest

Change-Id: Ibca0bd25bb008bbf9f3a635398d517dfba4b0efe