nessDB v3.0

Preface:

there are two types nodes: top buffer and leaf node
a) top buffer is unsorted array, size is 256KB
b) leaf is sorted array, size is 256KB too
c) top buffer fanout is 128, tree size is ~32MB
d) tree height is 2

1. insert

   a) if top buffer is full, flush the messages to children b) if one leaf is full, split it, and copy half messages to the new leaf if this top buffer fanout reaches 16, split the tree to two, and copy the half to another one. the split of the tree is very fast, it's just sequential read and append write.