[Backup Service] Concurrently scheduled tasks and warning the user of the pitfalls of sharing the same archive location.

Description:

Suppose there are multiple repositories sharing the same archive location.

Currently, mutual exclusion is enforced by cbbackupmgr at the archive level which means that two cbbackupmgr processes cannot perform a task on the same archive.

Suppose a user specifies the same archive location for two repositories falling in the following trap:

A user cannot perform two scheduled tasks concurrently between the two repositories. If multiple tasks scheduled are scheduled for the same time, the task which 'wins' the race to obtain the lock to the archive succeeds and the other tasks fail. 

If the user performs hourly tasks on the two repositories sharing the same archive location, half the tasks will fail.

What should happen instead:

The repositories should not conflict in this manner as they are separate entities.

This will be tackled by repository level mutual exclusion (MB-37660).

What is the improvement:

Currently it should be sufficient to make the user aware that using the same archive location between two repositories will result in the repositories clashing perhaps as a tool tip or in the documentation.

I suspect the user is quite likely to make this mistake of using the same archive location while using the backup service.