Make Corso backups restartable

[ashmrtn]

First backups can take a long time due to the amount of data to backup and throttling. Currently these backups only get minor benefits from kopia assisted incrementals, leading to long runtimes if a backup fails partway through and needs restarted.

Right now, restarting a backup will avoid reuploading previously uploaded data but will require enumerating all items again. Since item enumeration can be time-consuming this draws out backup time

Corso should be enhanced to have a way to restart backups that fail partway through. The enhancements should take into consideration things like liveness (as an extreme example, ensuring Corso will make progress and eventually generate a backup if it crashes after every 5min)

This ticket provides tracking information for individual items that need to be completed to implement restartable backups

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Basic features

• fetch enough information to make a details.ItemInfo entry for an item during enumeration #2854
• persist enumerated item pages to kopia #2855
• feed persisted enumerated item pages back into GraphConnector #2856
• use persisted item pages to source items to lookup during backup #2857
• recreate details.ItemInfo entries based on persisted item pages #2859
• define a state machine for Corso backups #2860
• find and reuse the most recent next or delta token from persisted item enumeration pages #2863

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Testing

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Phase 2

• switch to using stable IDs for Exchange items #2861
• Allow backups to resume in phases after item enumeration #2862