Rework StatsGatherer

[olly]

Try to replace the StatsGatherer? mechanism with something easier to understand and maintain.

[richard]

StatsGatherer? and StatsSource? no longer exist, and the code is much clearer I think, so I'm marking this as fixed.

Instead of the old mechanism, we now create a Stats object and pass it to the MultiMatch? constructor. This passes the object to each of the submatchers, calling prepare_match() on them to populate the Stats object with the statistics for the local set of collections. If we're running the search locally (ie, MultiMatch? is called from Enquire, not from RemoteServer?), the same statistics are then passed to the MultiMatch::get_mset() call. If we're running the search as part of a larger remote search (ie, MultiMatch? is called from RemoteServer?), the local statistics are sent off, merged with any other statistics, and the global statistics are then received from the client: the MultiMatch::get_mset() call is then called with the global statistics.

The flow of data here is reasonably straightforward and uncomplicated, so I think this is now maintainable.