cbbackupmgr: better logging when an incremental backup for a vbucket is rolledback to seqno 0

When using the cbbackupmgr, there are times when an incremental backup is performed, the backup for a vbucket starts back from seqno 0. There are legitimate reasons to do so, for example, when an incremental backup is performed on a vbucket that has witnessed delete operations during the time between now and the previous backup, and the metadata purge interval has already passed, thus requiring we start back from seqno 0. We would like to see better logging around that area to make it easier to identify what the issue is.