Add statistics collector utility

Summary:
1. Add statistics collector which collects statistics periodically at a period specified by caller.
2. Added unit-test for StatsCollector

make rocksdbjava
make test

Reviewers: haobo, yhchiang, sdong, dhruba, rsumbaly, zzbennett, swapnilghike
Reviewed By: yhchiang
CC: leveldb

Differential Revision: https://reviews.facebook.net/19215