Change the way moving messages between folders in the database works

Previously, when moving a message, the entry in the `messages` table was rewritten to reference the destination folder. Then a "deleted" message entry was created referencing the source folder.
This commit changes the move code to create a new entry for the message in the destination folder and then changes the original message entry to be a "deleted" entry.
It's a first step towards being able to use a message's database ID instead of the server ID in "pending commands".

This also continues the project of moving database-related code to the `storage` module. There should now be enough scaffolding in place to make moving code from `LocalStore` and `LocalFolder` relatively painless.