[Java] Make read benchmarks print out found / not-found information.

Summary: Make read benchmarks print out found / not-found information.

Test Plan:
make rocksdbjava
make jdb_bench
cd java
./jdb_bench.sh

Reviewers: haobo, sdong

CC: leveldb

Differential Revision: https://reviews.facebook.net/D18699
[Java] Refined the output of Java DbBenchmark.
Merge branch 'master' of github.com:facebook/rocksdb into show-reads