BPlusTree: Style cleanup only, no functional change.
haiku_loader: Reuse BPlusTree implementation of the BFS add-on.

Instead of having an almost exact, albeit read only, duplicate of the
implementation.
BPlusTree: Fix GCC4 false positive of possible unintialized use.
BPlusTree: Fix fCurrentKey in backward TreeIterator traversal.

When reaching the next node the current key should be set to the next
valid index within that node (0 for forward and NumKeys() - 1 for
backward). This did not cause any harm as BFS uses forward traversal
only.