Relazify the database if the plugin version has changed

[stuarth]

Often the lazified database format changes between database versions. Since the user cannot force the database to be re-lazified without performing a clear-and-rescan this can be somewhat inconvenient.

The plugin should detect it's been upgraded and completely remove and replace the lazified information in the database.

[stuarth]

Two ideas for how to do this:

1. Store the plugin version in the metainformation table in the database;
2. Store the plugin version in a new preference (that the user doesn't see - it won't appear on the plugin settings page).

Either way it would work in the same way:

1. In the rescan callback (which is also called when the plugin initialises), it reads the last plugin version from the preference/database.
2. It then compares that with the current plugin version (the subversion tag $Revision$ will be used to auto-populate the revision of the plugin).
3. If they are the same then the current behaviour continues - anything not yet lazified is lazified, but anything already in the database is left alone.
4. If the versions are different then the lazification has to be replaced. It proceeds as it currently does but doesn't ignore already lazified entries.

[stuarth]

As the database lazification is changing in milestone:2.2.2 I'll also get this into that version - it's a pretty straightforward change.

Having looked at the metainformation approach I believe it's neater to just use a preference.

[stuarth]

With this enhancement, the re-lazify database button on the plugin settings page should also forcibly relazify.

[stuarth]

This has been done in r201. On that basis I'm closing this as implemented.