Preserving sandbox deleted task history (part 2: state machine).

Bugs closed: AURORA-261

Reviewed at https://reviews.apache.org/r/19648/
Fix javadoc generation

Testing Done:
./gradlew javadoc
xdg-open dist/docs/javadoc/index.html

Bugs closed: AURORA-297

Reviewed at https://reviews.apache.org/r/19806/