Move limting and skiping of results into class method

To speed up testing, the skiping and limiting of results
has been moved into a class method. Enabling testing of
the limiting and skipping code without excuting a full
query
I suspect this will still have the CDTDatastore etc. created from its enclosing context -- move it outside that context so it doesn't have extra work per test. This might be a new top level context/describe block.