[PiTR] [Collections] Item count wrong after dropping collection

What's the issue?
When using PiTR, the item count appears to be incorrect in that it accounts for items that exist in dropped scopes/collections.

Steps to Reproduce
1) Spin up a single node cluster
2) (optional) limit the number of vBuckets to one
3) Create a non-default collection
4) Load some data into it
5) Allow time for data to be persisted
6) Drop the collection
7) Observe that the item counts in the UI are wrong (they account for the items in the dropped collection)
8) Observe that the item count reported as 'curr_items' by cbstats is incorrect (matches the UI)

Expected Behavior
The item count should not account for items in dropped scopes/collections when PiTR is enabled.