Rename folder 1/2

This MR is the first of two which add support for Renaming folders to siad. It adds a Rename method to the siadirset.

The workflow is as follows:

1. lock the set to prevent dirs from being opened
2. lock already opened dirs to prevent other threads from writing to disk
3. rename the folder on disk
4. if successful rename the already opened dirs in memory
5. unlock the dirs and set