Move away from proto serialization for RocksDB keys in Ozone

Currently there are a few places where we rely on protobuf serialization to convert the key type to byte array for a RocksDB column family 'key'. However, from the proto documentation it looks like the serialization of proto is unreliable (especially across versions). If the byte[] got from proto serialization changes, this will be a problem since old keys will be unreadable.

Thanks to arp who helped unearth this problem.