S3Guard import can OOM on large imports

I know I'm closing ~all S3Guard issues as wontfix, but this is pressing so I'm going to do it anyway

S3guard import of directory tree containing many, many files will OOM. Looking at the code this is going to be because

• import tool builds a map of all dirs imported, which as the comments note "superfluous for DDB". - cut
• DDB AncestorState tracks files as well as dirs, purely as a safety check to make sure current op doesn't somehow write a file entry above a dir entry in the same operation

We've been running S3Guard for a long time, and condition #2 has never arisen.

Propose: don't store filenames there, so memory consumption goes from O(files + dirs) to O(dirs)

Code straightforward, can't think of any tests