[Prospective] MNT: consolidate different Entry traversal use cases in Visitor

[shilorigins]

We re-implement the logic of traversing an Entry graph multiple times in our codebase, performing a different action at each node or tracking different metadata. These use cases could potentially be streamlined by implementing an EntryVisitor class containing the common logic, with subclasses performing the different specific functions.

Possible visitor subclasses:

• FillUUIDVisitor
• SearchVisitor (for FilestoreBackend / TestBackend)
• SnapVisitor

[tangkong]

Or possibly a visitor + post-visit hook structure? I realize that's the normal way you handle graph/tree traversal in school