public void stop() {
    log.debug("closing the db");
    if (db != null) {
      db.close();
    }
    log.info("db is closed.");
  }
get the value from the key. This key will be serialized to bytes using the
serde defined in <i>systems.system-name.samza.key.serde</i>. The result
will be deserialized back to the object using the serde in
<i>systems.system-name.samza.msg.serde</i>. If the value does not exist in
the db, it return null.

@param key the key of the value you want to get
@return deserialized value for the key
        Returns null, if the value doesn't exist
samza-kv-rocksdb/src/main/java/org/apache/samza/storage/kv/RocksDbKeyValueReader.java