implement several b-tree structures in c++.

BoltDB源码分析

1. using mmap file to manange page.
2. tx->commit->allocate(tx->DB->freepage)

• page(meta/freelist/data) management
• gtest/glog capability
• node management
• transaction
• bucket(B+Tree) management
• unit test
• performance test

• better memory management (avoid some copy)
• palm
• transaction manager