Re-add SQLite support

[snowteamer]

Closes #1484

Summary of changes

• Updated Node version to v18 LTS (was necessary to get FormData and Blob support in backend tests without pulling in new dependencies).
• User file uploads are now stored as blobs in Chelonia DB using blob:${hash} keys, whereas previously they were always stored in the ./data folder. This means that file uploads will no longer be persisted unless persistence is actually enabled.
• When set, GI_PERSIST can now be either fs or sqlite.
• In production, persistence is always enabled and defaults to fs if GI_PERSIST is not set or empty.
• A name: prefix is now used in namespace keys instead of name=.
• A head: prefix is now used in contract log head keys instead of a -HEAD suffix.
• The 'backend/db/readFile', '-writeFile', and '-writeFileOnce' selectors have been removed.
• The backend/database.js module must now be initialized before use.
• Storage unit tests have been added in test/database.test.js. These tests apply to both the fs and sqlite backends in a single run, so that you do not have to do multiple test runs with different GI_PERSIST values.

[taoeffect]

Great work @snowteamer! Some more minor issues and after these are addressed I think it should be good to go.

[taoeffect]

What was the verdict on adding a third column for metadata for dealing with lastModified header for fs?

[snowteamer]

That turned out to be a bit complex because lastModified has to be stored somewhere and kept up-to-date for all three storage backends, and also because we want a consistent API regardless the data type but only want that field for files.

[taoeffect]

This would read the entire database contents into memory, right? That seems like an extreme amount of data to read into memory (potentially hundreds of gigabytes worth). What is this function for?

[taoeffect]

Copying a note from @snowteamer from Slack about what this is:

These are database migration helpers, which I was planning to maybe use either in the preloading code or in chel migrate.
I've left them here so that they can be unit-tested in database.test.js and reviewed, but yeah actually they are not used (at least yet).

[taoeffect]

Nice work @snowteamer! I left a few comments.

Also, I tried testing this with GI_PERSIST=sqlite grunt dev, and ran into two problems:

1. I couldn't load the site, neither on port :3000 nor :8000
2. I didn't see the message [chelonia.db] Preloaded ${newKeys.length} new entries in the console either

This is (some of) the output I got:

Running "backend:relaunch" task
backend: forking...

Running "keepalive" task
[Browsersync] Proxying: http://127.0.0.1:8000
[Browsersync] Access URLs:
 ----------------------------------------
       Local: http://localhost:3000
    External: http://192.168.100.201:3000
 ----------------------------------------
          UI: http://localhost:3002
 UI External: http://localhost:3002
 ----------------------------------------
[Browsersync] Watching files...
[SBP WARN]: registering unsafe selector: 'chelonia/db/get' (remember to lock after overwriting)
[SBP WARN]: registering unsafe selector: 'chelonia/db/set' (remember to lock after overwriting)
[SBP WARN]: registering unsafe selector: 'chelonia/db/delete' (remember to lock after overwriting)
Connected to the groupincome.db SQLite database.
[SBP WARN]: registering unsafe selector: 'chelonia/db/get' (remember to lock after overwriting)
[SBP WARN]: registering unsafe selector: 'chelonia/db/set' (remember to lock after overwriting)

[taoeffect]

Approved! Great work @snowteamer!!