[Java] Add multiget JNI bindings

Summary:
1. Add multi-get JNI bindings
2. Unit-tests for multi-get

make rocksdbjava
make sample

Reviewers: haobo, yhchiang, sdong, dhruba, rsumbaly, zzbennett, swapnilghike

Reviewed By: yhchiang

CC: leveldb

Differential Revision: https://reviews.facebook.net/D18369