Introduce page "id", which is a page "index" plus a non-negative number.

The index is ranging from 0 to the number-of-page-minus-1, while the id
is used to termine related "buddy" block.
add rbt_search_variant which is able to search an element greater/less than a given key
Implement lm_unmap() features about unmapping leading or trailing partion
of mapped block. Unmapping middle portion of mapped area is not yet implemneted.

This change is not tested yet.