doveadm backup (rbox-> mdbox) stops if mailbox index has invalid entries

[jrse]

If the rbox mailbox index has invalid entries e.g. a reference to a non-existent mail object, the doveadm backup process to another mailbox format like mdbox will fail.

Fix this to the following behavior:

• backup all valid mail objects to the other mailbox format
• print error for all invalid index entries.

Workaround: do a doveadm force-resync before the backup if the index is invalid.

Steps to reproduce:

list objects:
doveadm rmb ls -u t1 -

delete a object with
doveadm rmb delete

do backup to mdbox

doveadm -c /../dovecot-2.2.21/etc/dovecot/dovecot.conf -v -F backup -f -u t1 mdbox:/...dovecot-2.2.21/var/mail/mdbox/
=>
dsync(t1): Warning: Errorcode: -2 cannot get x_attr from object 241c9c1d0486505b406200009c60b9f7, process 26972
dsync(t1): Error: rbox /../dovecot-2.2.21/var/mail/rbox/t1/mailboxes/INBOX/rbox-Mails: Unexpectedly lost uid=60
dsync(t1): Error: Exporting mailbox INBOX failed: Can't lookup GUID for UID=60: Internal error occurred. Refer to server log for more information. [2018-07-19 15:18:20]