use MemoryPool instead of allocator, fix examples

[zxcv05]

MemoryPool is more performant and efficient than using a generic allocator, especially for lots of create()s and destroy()s of only a single type (Node(K, V, D) in this case)

also refactored part of the BPlusTree struct for easier readability and nicer qualities like BPlusTree(K, V, D).Node instead of having detached BPlusTree(K, V, D) and Node(K, V, D) (they are intrinsically linked)