Fix the migration feature that filters unreferenced slabs

[fxamacker]

Recently, migration programs in onflow/flow-go added a flag to filter out old unreferenced slabs and onflow/atree added some functions to support that. However, some of the old unreferenced slabs are not filtered.

This issue only affects this new migration feature in onflow/flow-go:

• Add a migration which detects and filters out unreferenced slabs flow-go#5653
• 🧵 https://discord.com/channels/613813861610684416/1232975765847543839/1233068490030846023

More info:

• https://github.com/dapperlabs/cadence-internal/issues/187

Suggestion

After looking into this today, it is clear that traversal needs to go deeper to handle nested storage id inside another storable such as Cadence SomeValue. Update the code to do this and add more tests.