[BUG]Fix B-Tree metadata, range function for auto_scaling

[charles-typ]

Describe the bug

We want to store B-Tree slot_begin and slot_end into it's metadata, so we need to make it a map<std::string, std::string> instead of a single string. The B-Tree range function does not support auto scaling, need to add implementation for it.
We also want to add script for user to generate B-Tree automatically, which as follows:
1.Currently there is separated code for generating B-Tree keys, should add a script to generate the keys automatically
2.Add reference to the repo of the alpha words.