Migrates the Map database to the new V3.x format,
that uses UUIDs  instead of player names to designate
players (among other improvements). Migration runs in
a separate thread, therefore its progress can only be
watched from the server console.

The migration will run the following steps.

- Checking if there are files to migrate. If not, the
   migration stops.
- Checking if there are backups from a previous
   migration. If there are, the migration stops.
- Loading the old map and poster data to memory.
- Backing up old files and new files, to the
   §7backups_pre-v3§r and §7backups_post-v3§r sub-
   directories respectively. Backup's integrity are
   checked using file size and SHA1 checksum. If
   integrity could not be proved, the migration stops.
- Retrieving the UUIDs of the players from Mojang's
   servers.
- Checking if some UUIDs could not be retrieved.
   If there are, this means some of your players may
   have changed names before the migration started.
   The plugin will therefore try to retrieve them
   specifying a time, back when usernames could not
   be changed. If some names could still not be matched
   to their UUIDs, then these are probably non-paid
   accounts. If no UUID has been retrieved at all,
   the migration stops.

§c--- From this step, changes to disk will be made,§r
§c--- and you will have to use backups if you want to§r
§c--- revert back from before the migration started.§r

- Merging the old map data with the new one, if there
   is any (which can be the case if your player started
   to use newer versions of ImageOnMap before the
   migration started).
- Saving all this merged map data to disk.
- Removing the old map data from their former files,
   leaving only the data that could not be migrated
   due to usernames that could not be matched to their
   UUIDs. Original data is still present in the
   appropriate backup directory, just in case.

Note that this plugin will NEVER delete nor overwrite
any backup directory. Moving or deleting these backups
is left to the administrator's responsibility.