Backup client should disable third party plugins before starting backup

Ideally the backup client should disable third party plugins before it initiates a backup and re-enable them after the backup is finished.

An idea would be setting UPM to safe mode before the backup and unsetting it afterwards.

We should also provide instructions in the DIY backup if possible.

Customers can get around this issue ("One or more database connections were not closed within the allotted timeout") if they follow this KB: https://confluence.atlassian.com/display/STASHKB/Backup+client+fails+due+to+the+error:+One+or+more+database+connections+were+not+closed+within+the+allotted+timeout