Enable selective subtree pruning for Exchange backups

[ashmrtn]

Exchange is not currently eligible for selective subtree pruning during hierarchy merging because it doesn't provide enough LocationPath information during backup. Once support for the following is added Exchange will automatically begin using selective subtree merging. That should hopefully help reduce incremental backup time for large Exchange hierarchies when things haven't changed

Issues required for support:

• Persist Exchange LocationPaths as Corso metadata during backup #4129
• Don't create empty Exchange BackupCollections #4130

[ryanfkeepers]

@ashmrtn is this already in progress?

[ashmrtn]

this is not in progress. Not sure we have good data on how much this will help as we don't have good numbers on the number of folders in exchange backups as far as I know