xapian-compact functionality should be available from the C++ API

[richard]

The ability to merge and compact databases efficiently would be a useful addition to the C++ API (and the language bindings), so it would be good to move most of the implementation xapian-compact into the core, and change xapian-compact to just be a simple interface to this.

The first step is probably to refactor xapian-compact, such that it's not mainly a single massive function: I've made a start on this, and the patch will be attached to this bug shortly.

I'm happy to work on this, and don't think it's very much work, but Olly says that there are a few outstanding issues he needs to fix in xapian-compact, so I'll leave this bug assigned to him until then.

[richard]

Some refactoring of xapian-compact

[richard]

It would also be useful to be able to check databases from the C++ API and language bindings (ie, to bring the implementation of xapian-check into the core).