Use PRAGMA user_version instead of a version table

[lemon24]

PRAGMA user_version can be used within transactions: https://stackoverflow.com/a/18550663 (I also tested myself). The default value is 0.

Pros:

• no special table
• that's what it was made for
• might be faster than a table (e.g. we might be able to do it before every transaction if needed – I'm somewhat worried a long running process could corrupt the database data because the schema version was updated since it started)

Cons:

• not portable (but we didn't really care about this anyway)
• assumes we "own the database" (which is probably OK)
• we won't be able to get rid of the old version check until 2.0 (because we still have a bunch of migrations that use it, and we'll drop the in 2.0)

[lemon24]

This took about 2.5h to implement.