[JNI] Add an initial benchmark for java binding for rocksdb.

Summary:
* Add a benchmark for java binding for rocksdb.  The java benchmark
  is a complete rewrite based on the c++ db/db_bench.cc and the
  DbBenchmark in dain's java leveldb.
* Support multithreading.
* 'readseq' is currently not supported as it requires RocksDB Iterator.

* usage:

  --benchmarks
    Comma-separated list of operations to run in the specified order
        Actual benchmarks:
                fillseq    -- write N values in sequential key order in async mode
                fillrandom -- write N values in random key order in async mode
                fillbatch  -- write N/1000 batch where each batch has 1000 values
                              in random key order in sync mode
                fillsync   -- write N/100 values in random key order in sync mode
                fill100K   -- write N/1000 100K values in random order in async mode
                readseq    -- read N times sequentially
                readrandom -- read N times in random order
                readhot    -- read N times in random order from 1% section of DB
        Meta Operations:
                delete     -- delete DB
    DEFAULT: [fillseq, readrandom, fillrandom]

  --compression_ratio
    Arrange to generate values that shrink to this fraction of
        their original size after compression
    DEFAULT: 0.5

  --use_existing_db
    If true, do not destroy the existing database.  If you set this
        flag and also specify a benchmark that wants a fresh database,  that benchmark will fail.
    DEFAULT: false

  --num
    Number of key/values to place in database.
    DEFAULT: 1000000

  --threads
    Number of concurrent threads to run.
    DEFAULT: 1

  --reads
    Number of read operations to do.  If negative, do --nums reads.

  --key_size
    The size of each key in bytes.
    DEFAULT: 16

  --value_size
    The size of each value in bytes.
    DEFAULT: 100

  --write_buffer_size
    Number of bytes to buffer in memtable before compacting
        (initialized to default value by 'main'.)
    DEFAULT: 4194304

  --cache_size
    Number of bytes to use as a cache of uncompressed data.
        Negative means use default settings.
    DEFAULT: -1

  --seed
    Seed base for random number generators.
    DEFAULT: 0

  --db
    Use the db with the following name.
    DEFAULT: /tmp/rocksdbjni-bench

* Add RocksDB.write().

Test Plan: make jbench

Reviewers: haobo, sdong, dhruba, ankgup87

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17433