Rename SnapshotProducer::manifest_file #2529

It was unclear to me what manifest_file function is doing from its API signature. Maybe it is lingering with an old name from when we only had simple appends.

Ultimately, it is building a list of manifest files that will be part of the new snapshot / manifest list. I think we should consider renaming it to something like build_manifest_file_list.

Originally posted by @dannycjones in #2367 (comment)