Reduce memory dependence; persist to multiple files

[dev-dull]

Currently, the entire database lives in memory and is persisted to a single file on disk. Consider making each registered id be persisted to a separate file. By loading/unloading the file (containing the data from a single id) on user request(s) we will likely make the memory footprint a lot smaller.

Pros:

• Smaller, more manageable files.
• Data from each id is isolated making them easier to move around and share
• Data is loaded/saved based on ID, reducing how
• Its harder to corrupt the entire data set just because of one unclean shutdown.

Cons:

• Although we already persist to disk with every request, PyXIE will be doing a lot more reads from disk, making things slower
• The DATABASE_FILE configuration would change to something like DATA_PATH which some may consider a breaking change, so a major version bump would be recommended.
• A data migration process from the single file database to multiple a multi-file database would need to be defined.

This ticket may lend itself well to #25 where each thread that PyXIE runs would receive a request from a queue, then loads the correct file for the id, and processes the request. Unclear how well that strategy would work with Flask, however.