Fix #111 Drop::sendAndForget can return a false value
Move DropResult and DropResultPair to a new class
Update DropMessage<T> to DropMessage<? extends ModelObject>
Remove unneeded lines
Add buffered data classes
Add identity id
Update function name
Add new test function testing check function
Move functions from Drop to DropController
Merge branch 'drop-refactoring' into acknowledge
Change AcknowledgeID::model_object from string to class<T>
Merge branch 'buffered-data' into acknowledge
Add new empty Drop class
Add test function