Fix BTree for balance splitting.

[TKONIY]

A balance splitting for a full node with m max entries is as follows:

For example the Inner Node, m must be even (to support preemptive splitting), thus the number of the key is count=m-1, while count is odd. The node is split into two nodes each with floor(count/2) node, and the center node is inserted into the parent.

In previous implementation, the inner node is split into nodes with count-ceil(count/2)-1 and ceil(count/2) keys. For example, when m=4, the full inner node with 3 keys is split into nodes with 3-ceil(3/2)-1 = 0 and ceil(3/2) = 2 keys. Which is imbalance and conflict with to BTree's definition, that "Every internal node has at least ⌈m/2⌉ children".