Support time binding comparisons inside HAVING clauses

[rogerlucena]

If we extract a time binding inside the WHERE clause it would be useful to be able to use it for comparisons inside the HAVING clause, creating conditions around that binding. This PR comes to add support for this.

To illustrate, the query below is now possible:

SELECT ?p, ?time
FROM ?test
WHERE {
    /u<peter> ?p AT ?time ?o
}
HAVING ?time < 2016-03-01T00:00:00-08:00;

On which the time binding ?time extracted with the AT keyword can now be compared to a time literal (2016-03-01T00:00:00-08:00 above) inside the HAVING clause.

You can also combine it with ID bindings to create more complex conditions inside the HAVING clause:

SELECT ?s, ?p, ?p_id, ?time
FROM ?test
WHERE {
    ?s ?p ID ?p_id AT ?time ?o
}
HAVING (?p_id < "in"^^type:text) AND (?time > 2016-02-01T00:00:00-08:00);

NB: The comparisons with a time literal are not lexicographical, they indeed have the semantics of a time comparison: we consider the time zone, for example (as one should expect).

So, if you use a condition like ?time = 2012-03-10T00:00:00-08:00 in your HAVING clause, you may also get triples in your query result with the ?time binding being 2012-03-10T09:00:00+01:00 (which is equivalent to the timestamp requested in the condition, being in another time zone).

For more examples on this, please refer to the new tests added in planner_test.go and expression_test.go.