Upgrading from Dovecot Pro 3.1.x to Dovecot Pro 3.1.2

@fs_defaults = nfs and @fts_fs_defaults = nfs in earlier versions did not include the metawrap fs. This meant that any objects written with the configuration did not include any metadata. When using dovecot_config_version = 3.1.2 the metawrap fs is added, which adds the metadata.

DANGER

This change makes the existing objects unreadable. If you have stored any mail or index objects with the old @fs_defaults=nfs or @fs_fts_defaults=nfs configuration, you will need to perform data migration.

WARNING

If the metawrap fs is not added, objects will not have any metadata. This can cause problems mainly if index files ever need to be rebuilt, which happens sometimes due to various reasons. Most importantly the email's "received timestamp" (IMAP INTERNALDATE) will be lost, and it'll be assigned as 1970-01-01.