Draft: Fix SD Meta Data

• SelfDescriptionMetadata.java:
  • Bugfix: Since this class extends the Serializable interface via the SelfDescription class, provide a serial version UID.
  • Bugfix: Declare member variable "selfDescription" as transient to fulfill contract of serializable interface.
  • Bugfix: Instead of calling OffsetDateTime.now() twice (and thus getting two almost identical timestamps), create a single timestamp and pass it in twice via the new private convenience constructor.
  • Clean-up: Also use the new private constructor to get rid of duplicate code (calls to method calculateSha256AsHex() and initialization of selfDescription member).
  • Clean-up: Now that there is only a single call to method HashUtils.calculateSha256AsHex() as well as to the OffsetDateTime.now() method left, revert static import into an ordinary import.
  • Clean-up: Remove unused ArrayList import.
  • Clean-up: Add missing JavaDoc comments.
  • Clean-up: Apply consistent spelling for "self-description".