Fix the unit test failure in devbox

Summary:
My last diff was developed in MacOS but in devserver environment error occurs.

I dug into the problem and found the way we calcuate approximate data size is pretty out-of-date. We can use table properties to get more accurate results.

Test Plan: ran ./table_test and passed

Reviewers: igor, dhruba, haobo, sdong

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16509