add the ability to clear completed data children from the tree.
add missing javadoc comments.
Add a cursor for traversing leaves of a restore item tree.
strip restore item down to what the restore engine needs, if something
more flexible is needed in the future it will be created then.
use RestoreWork in RestoreEngine
reduce RestoreEngine to a simple loop assist.
eliminate use of SplitReader
tidy up restore engine test cases.
Remove some unused information from RestoreEngine, making it simpler.
note that the progress counter is actually a read counter.