Implement lazy data fetch for Exchange items.
Use a new collection type to clearly denote
when items can be lazily fetched vs. requiring
eager fetch
**This PR changes how the read bytes stat is
updated.** Lazily fetched items will not
update the read bytes stat. This stat doesn't
appear to be used anywhere at the moment
For items that are deleted between the time
enumeration takes place and the time the data
for them needs fetched, the corso will:
* return an empty reader for the item
* not add the item to backup details
* delete the (empty) item from kopia on the
next backup
Manually tested deleting an item between
enumeration and data fetch
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#### Does this PR need a docs update or release note?
- [x] ✅ Yes, it's included
- [ ] 🕐 Yes, but in a later PR
- [ ] ⛔ No
#### Type of change
- [x] 🌻 Feature
- [ ] 🐛 Bugfix
- [ ] 🗺️ Documentation
- [ ] 🤖 Supportability/Tests
- [ ] 💻 CI/Deployment
- [ ] 🧹 Tech Debt/Cleanup
#### Issue(s)
* closes#2023
#### Test Plan
- [x] 💪 Manual
- [x] ⚡ Unit test
- [x] 💚 E2E
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