Renaming Trees

[theduke]

Use Case:

During asynchronous migrations, it's quite convenient to create a new tree and step by step copy data over from the old one.

Once finished, it would be convenient to drop the old tree and rename the new one.

This is only doable right now be keeping a mapping of concrete tree names, which is of course possible, but less convenient.

Proposed Change:

Add a Db::rename_tree(name) method.