Closes SUR-53
When creating a new context for subqueries or statement clauses, we used to have to clone any variables/values, and freeze the context, so that it could be used across threads and async boundaries.
Now with the new executor pattern for parallel queries, we can pass references instead, improving performance by removing unnecessary cloning of values.
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.
