add private member

Add primaryPageNumber and overflowPageNumber to count the exact number
of primary page and overflow page.

1) When a merge operation happens, primary page number subtract 1
2) When a split operation happen, primary page number add 1
3) When a empty overflow page is used again, empty overflow page number
subtract 1 and overflow page number add 1
4) When a new overflow page is created, overflow page number add 1