Handle leaf node underflow

[girikuncoro]

TODO Handle LeafNode Underflow (merge or redistribution)

@param left: the smaller node
@param right: the bigger node
@param parent: their parent index node
@return the splitkey position in parent if merged so that parent can delete the splitkey later on. -1 otherwise

[girikuncoro]

Not yet merged but pushed to deletion branch https://github.com/girikuncoro/bplustree/blob/deletion/BPlusTree.java

[girikuncoro]

Will merge once all deletion is complete