Java Bindings: prevent segfault from double delete

Give BackupableDB sole responsibility of the native
object to prevent RocksDB from also trying to delete
it.
BackupableDB: don't keep a reference to the RocksDB object now that we have disOwnNativeObject
[Java] Makes DbBenchmark takes 0 and 1 as boolean values.

Summary:
Originally, the boolean arguments in Java DB benchmark only takes
true and false as valid input.  This diff allows boolean arguments
to be set using 0 or 1.

Test Plan:
make rocksdbjava
make jdb_bench
java/jdb_bench.sh --db=/tmp/path-not-exist --use_existing_db=1
java/jdb_bench.sh --db=/tmp/path-not-exist --use_existing_db=0
java/jdb_bench.sh --db=/tmp/path-not-exist --use_existing_db=1

Reviewers: haobo, dhruba, sdong, ankgup87, rsumbaly, swapnilghike, zzbennett

Reviewed By: haobo

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D18687
Merge pull request #153 from bpot/fix_jni_segfault

Segfault when using BackupableDB with Java bindings