First off, we don't block the GUI thread anymore for ClangCompleter (that was
always temporary). Secondly, now ClangCompleter will cache the data coming from
clang so that query-based filtering of members is fast.
This removes the need for a special overload for AddCandidatesToDatabase. Also,
the GetFuture function now provides a more sensible API with the list being
returned instead of accepted as an out parameter.
