nautilus does this when you drag and drop to overwrite a file:
1) create .goutputstream-XXXXXX to write to
2) fsync the fd for .goutputstream-XXXXXX
3) rename .goutputstream-XXXXXX to target file
4) close the fd for .goutputstream-XXXXXX
previously, doing this on s3fs would result in an empty target file
because after the rename, s3fs would not flush the content of
.goutputstream-XXXXXX to target file.
this change moves the FdEntity from the old path to the new path
whenever rename happens. On flush s3fs would now flush the correct
content to the rename target.
