Additional index restore function

[jrse]

Currently, index restore function determine all mails belonging to a mailbox via oldindex.mailboxGUID and xattr.mailbox_guid value.
For each found rados mail object the xattr.UID is read and a lockup with the uid is done in index.log and index.backup, if found entry is used to restore the index entry.

There are several other restore functions thinkable without using old index files to restore the mailbox index. E.g. by only using XAttr.

Task is to examine if it makes sense to implement alternative restore function. And if so, what additional information needs to be saved as xattr.

[jrse]

The current index rebuild function uses, additionally to the already available Metadata like oid, guid, data from if existent the old index file. Namely those are mail flags and keywords. Additionally, the current rebuild tries to restore the mail_cache from the old cache. For that it uses the old sequence number of the mail, which it currently tries to restore based on uid. To be able to restore the cache also, we would need to create and update a new Metadata attribute old_sequence.

[jrse]

We discussed the issue and came to the conclusion, that we should make it possible for the admin to configure which "optional" mail metadata should be saved and updated in rados.
For that a new issue will be created.