Meta server: implement "range truncate" to allow to create chunk boundary aligned holes in non striped files. The primary intention is to use such truncate with write append in sorter, to allow to discard sorted chunks. Optimize meta b+tree iteration / search by avoiding redundant binary search withing terminal b+tree nodes. Optimize chunk search in truncate by performing linear search if the first chunk position (offset) is close enough (less than 8 chunks) to the target position, otherwise do b+tree search.