Backups are not purged or used during restore correctly when nodes share a backup directory

[komlenic]

Consider two nodes in a project that share/use the same directory for backups (in one example this directory is an NFS mount on multiple servers): When remove_old_backups() is run, currently the node name is not considered, only the dates of the backups.

Additionally, the most recently dated backup is used for a restore - and this could theoretically not be the correct backup from the correct node, in a situation where nodes share a backup directory.

Fixing this issue involves segmenting backups that are considered for purge or use in a restore, by node name.