libtracker-data: Lighten up TrackerSparql struct

There is an internal struct carrying the parser state, this struct began small, but kept growing. This state is actually unneeded after parsing, so we can keep TrackerSparql smaller, and only allocate the state transiently during parsing.

Since TrackerSparql is the basis for TrackerSparqlStatement, this should help memory usage a tiny bit if many prepared statements are cached.