record base backups in backup model

[ashmrtn]

To make debugging easier we should record what backups/snapshots participated as bases (with their reason) in the current backup. This would allow us to trace the chain of incremental backups even if the log files for a run aren't available

[ryanfkeepers]

@ashmrtn is this already in progress?

[ashmrtn]

not in progress right now (at least not by me, usually I'll self-assign when I pick things up)