[Java] Add documentation on interdependency of dispose call of RocksObjects and Optimize StatsCollector

This diff contains two changes:

1) Add documentation on interdependency of dispose call of RocksObjects

make sample
make test

Reviewers: haobo, yhchiang, dhruba, rsumbaly, zzbennett, swapnilghike
Reviewed By: yhchiang
CC: leveldb

Differential Revision: https://reviews.facebook.net/D20379

2) Optimize statistics collector

make test

Reviewers: haobo, yhchiang, dhruba, rsumbaly, zzbennett, swapnilghike
Reviewed By: yhchiang
CC: leveldb

Differential Revision: https://reviews.facebook.net/D20265