vfs: fix writeback deadlocks and other bugs

- fix deadlock when cancelling upload
- fix double upload and panic after cancelled upload
- fix cancelation strategy of uploading files
    - don't cancel uploads if we don't modify the file
    - cancel uploads if we do modify the file
- fix deadlock between Item and writeback
- fix confusion about whether writeback item was being uploaded
- fix cornercases in cancelling uploads and removing files

vfs/file.go

@@ -558,6 +558,7 @@ func (f *File) Remove() (err error) {
 		if wasWriting {
 			// Ignore error deleting file if was writing it as it may not be uploaded yet
 			err = nil
+			fs.Debugf(f._path(), "Ignoring File.Remove file error as uploading: %v", err)
 		} else {
 			fs.Debugf(f._path(), "File.Remove file error: %v", err)
 		}