Introduce persister that uses a directory structure

Created by: LoveIsGrief

Related to #36 (closed)

Using the base64 SHA-256 of the shared file's absolute path, a folder structure of <muwireHome>/files/<hash[:6]>/<hash[6:>.json. The cut length of the hash was chosen pretty randomly, I admit, but the goal is reduce the number of json files in one folder and reduce the number of possible folders.

A migration from the files.json is also in this PR.

Advantages:

• json files are updated individually when a file has been hashed
• reduced write speed
• basically a hashmap so access is O(1)
• no extra library needed

Disadvantages:

• One JSON per file --> increased inode count
• No hash collision check
• unwieldy pathnames

Possible improvements:

• the pathnames are quite long so maybe a different hash algorithm can be chosen
• cut length was chosen randomly so maybe a clever calculation calculate the optimal length for a good balance between directory count and file count per directory
• another subdirectory level for less files per directory