Support persisting completed tasks.

[hongwen000]

Description

Persiste the completed tasks to a json database (based on lowdb).
The database location is in the Data directory returned by the env-paths package.
Implemented a simple caching policy to avoid keeping reading the database.

Related Issues

Fixes #645

Checklist:

• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Have you linted your code locally prior to submission?
• Have you successfully ran app with your changes locally?

Tested

• Persisting to db
• Reading from db
• Delete task from db
• Purge all tasks

[agalwood]

Sorry for replying to you now, your PR branch cannot be merged automatically due to conflicts, you need to re-update the dependency adaptation.