Remote backend should support use of Xapian::MatchDecider

[richard]

Currently, Enquire::register_match_decider() simply stores the values passed to it in the internals of the Enquire object. These values never get used.

Either register_match_decider() should be removed, or (more probably) the values should be used in the remote match case to allow match deciders registered with the server to be used.

For now, I've added a note in the documentation comment that this method effectively does nothing.

[olly]

This method anticipates remote backend support for MatchDeciders?, so removing it isn't helpful unless we come up with a better approach.

We don't seem to have a bug for lack of remote MatchDecider? support, so reusing this one.

The main difficulty I can see is that the MatchDecider?-derived object would need to be serialised across before the match and back again afterwards (assuming the subclass has data members). That's going to put extra requirements on user implemented subclasses of MatchDecider?, so this definitely can't go into 1.0.x.

[olly]

Thinking about this more, the approach of register_match_decider seems wrong - it would be better (for consistency if nothing else) to handle this like we do for Weight subclasses - add a method to the class which returns the name. We'll also need to allow the object to serialise and unserialise any state.

So I think deprecating the current Enquire::register_match_decider() would be best - I'm just building and will commit that change shortly. Since it doesn't actually do anything, it seems unlikely anyone is using it, so perhaps it could be dropped in 1.1.0 - for now I've marked it for removal in 1.2.0.