Enable answering RQL select queries with symbolic bindings
This introduces a version 2.0 of the rqlio protocol that is able to answer select RQL queries with symbolic bindings (with the names of the selected variables) instead of the positional rows. Because this breaks compatibility with the 1.0 protocol, this change warrants a new version. The previous version continues to work alongside the new one.
The end result is the ability to answer the query Any X WHERE X is CWEtype with { 'rows': [{'X': 101}, {'X': 102}, {'X': 103}, ...], 'variables': ['X'] } instead of [[101], [102], [103], ...]
In protocol version 2.0, when there are no variable names, the variables member is simply empty. In addition, the value of the 'rows' member of the response object is exactly the same as the total response in the previous protocol, which should help write fallback code in client libraries.