Segfault when using BackupableDB with Java bindings

During GC the C++ RocksDB object is being deleted twice causing a segfault. Once by the RocksDB finalizer which calls `delete` on the underlying C++ RocksDB object directly and also via `delete` being called via the BackupableDB finalizer.

**RocksDB GC'd**:
```
C  [librocksdbjni.so+0x89c38]  Java_org_rocksdb_RocksDB_dispose+0x28
j  org.rocksdb.RocksDB.dispose(J)V+0
j  org.rocksdb.RocksDB.dispose()V+12
j  org.rocksdb.RocksDB.close()V+1
j  org.rocksdb.RocksDB.finalize()V+1
J 428 C1 java.lang.ref.Finalizer.runFinalizer(Lsun/misc/JavaLangAccess;)V (62 bytes) @ 0x00007fe
78120d444 [0x00007fe78120cec0+0x584]
J 366 C1 java.lang.ref.Finalizer.access$100(Ljava/lang/ref/Finalizer;Lsun/misc/JavaLangAccess;)V
 (6 bytes) @ 0x00007fe7811e189c [0x00007fe7811e1840+0x5c]
j  java.lang.ref.Finalizer$FinalizerThread.run()V+45
```

**BackupableDB GC'd:**
```
C  [librocksdbjni.so+0x12788d]  rocksdb::BackupableDB::~BackupableDB()+0x6d
C  [librocksdbjni.so+0x1278c1]  rocksdb::BackupableDB::~BackupableDB()+0x11
j  org.rocksdb.RocksDB.dispose(J)V+0
j  org.rocksdb.RocksDB.dispose()V+12
j  org.rocksdb.RocksDB.close()V+1
j  org.rocksdb.BackupableDB.close()V+8
j  org.rocksdb.BackupableDB.finalize()V+1
J 371 C1 java.lang.ref.Finalizer.runFinalizer(Lsun/misc/JavaLangAccess;)V (62 bytes) @ 0x00007f88
b91e3294 [0x00007f88b91e2d00+0x594]
J 370 C1 java.lang.ref.Finalizer.access$100(Ljava/lang/ref/Finalizer;Lsun/misc/JavaLangAccess;)V 
(6 bytes) @ 0x00007f88b91e299c [0x00007f88b91e2940+0x5c]
j  java.lang.ref.Finalizer$FinalizerThread.run()V+45
```

It seems like the appropriate behavior would be to have the nested RocksDB (db_ in Backupable.java) instance's finalizer be a no-op but I'm not sure the best way to accomplish this.