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
Merge pull request #153 from bpot/fix_jni_segfault

Segfault when using BackupableDB with Java bindings