The intended use case is to enable migration programs in onflow/flow-go to fix
broken references.  As of April 2024, only 10 registers in testnet (not mainnet)
were found to have broken references and they seem to have resulted from a bug
that was fixed 2 years ago by onflow/cadence#1565.

A broken reference is a StorageID referencing a non-existent slab.

This commit adds FixLoadedBrokenReferences(), which traverses loaded slabs and
fixes broken references in maps.  To fix a map containing broken references,
new function replaces broken map with empty map having the same StorageID and
also removes all slabs in the old map.

Limitations:
- only fix broken references in map (this is intentional)
- only traverse loaded slabs in deltas and cache

IMPORTANT: This should not be used to silently fix unknown problems. It
should only be used by migration programs to fix known problems which
were determined to be appropriate to fix.

Co-authored-by: Bastian Müller <[email protected]>

Currently, calls to this function can be limited by migration
programs by specifying the 9 testnet accounts affected by 10
registers with broken references on testnet.

This commit allows callers to implement additional restrictions,
so calling FixLoadedBrokenReferences for the affected 9 testnet
accounts can be even more limited at the callers discretion.

In practice, this change is not expected to produce different
migration results because full migration tests before this change
correctly fixed the 10 known registers in the 9 testnet accounts.

This commit added predicate func(old Value) bool to
PersistentSlabStorage.FixLoadedBrokenReferences() to control
whether to fix a atree.Value containing broken references.

Also modified PersistentSlabStorage.FixLoadedBrokenReferences()
to return fixed storage IDs and skipped storage IDs.  Both returned
values are of type map[StorageID][]StorageID, with key as
root slab ID and value as all slab IDs containing broken references
connected to the root.

Also added more tests and improved existing tests.

This commit adds GetAllChildReferences() which essentially wraps
an existing internal function.

…ences

Add PersistentSlabStorage.GetAllChildReferences()