Speed up write?

### Proposed solution

Speed up write speed via network.

Right now beets writes the same file twice (tested on `.flac` but probably it's true for all format). The first time it had happened when files is moved into library. It works as expected via temporarry name. Anyway, when it edits tags, it reuploads the whole file with edited tags.

I see this behaviour quite well when upload heavy `.flac` into library via WebDAV by investigating `.davfs.*` files in the distination folder via SSH to the NAS.

Rework editing of tags from save full file, into append or truncate and replacing only tag part at the end of the file.

Reproduce the write behavior with a heavy .flac transferred through WebDAV, inspecting the .davfs.* files on the NAS as described. Trace beets' tag-editing path and determine whether the format permits replacing only the tag portion; done means tag edits no longer reupload the entire file while preserving correct files and tags.

Written by the indexing model from the issue text.