(you don't have to strictly follow this form)

Use case

Sometimes, for really sophisticated scenarios it can be hard to control the amount of in-progress backups run externally. For the purpose of simpler control we can introduce server-level setting which disallows running concurrent backups of any type.

Something like this in config.xml (enabled by default):
<allow_concurrent_backups>true</allow_concurrent_backups>

Describe the solution you'd like

It's important that backups can be executed in distributed way on multiple servers. Need to take it into account.

Describe alternatives you've considered
External check of system.backups table, but it can be complex.