Simplify time management

[odrotbohm]

Simplify TimeService subsystem to the following:

interface TimeService {

  /**
   * Returns the current application time. This will be the time of the system 
   * the application is running on by default but can be adjusted by calling {@link #forward(…)}.
   * …
   */
  LocalDateTime getTime();

  /**
   * Forwards the current time with the given {@link Duration}. Calling the 
   * method multiple times will accumulate durations.
   * …
   */
  void forward(Duration duration);

  /**
   * Returns the current offset between the real time and the virtual one 
   * created by calling {@link #forward(…)}.
   * …
   */
  Duration getOffset();
}