Instead of using static futures when processing a query in parallel, we now use a new executor model, which allows us to process futures which make use of references. As a result, we can remove the need to store each statement in an Arc.
Due to a bug in msgpack-rust (https://github.com/3Hren/msgpack-rust/issues/86), in order to have correct serialization<->deserialization of data, we need to ensure all fields are serialized even when empty. This will affect the output size of the data, but will ensure that once serialized, the data is deserialized without issue.
