Option to avoid concurrent backups

Problem/Motivation

Running multiple backups simultaneously could be very problematic for smaller servers.

Proposed resolution

Provide an option (maybe a default?) that allows multiple backups to run concurrently.

Remaining tasks

Work out a solution.

User interface changes

Provide an option (maybe a default?) that allows multiple backups to run concurrently.

API changes

TBD

Data model changes

TBD