This is the last part of #419 - with the move to supporting mutable databases by default, the inspect-data mechanism currently in use no-longer makes much sense.

The one optimization I think it's worth keeping for databases opened in immutable mode is the cached table counts. I think datasette inspect should cut down to only counting the rows in the tables - the other things done by inspect (figuring out columns, foreign key relationships, FTS etc) should all be fast enough that they can be reliably performed at runtime even against large databases.

If performing them at run-time has performance issues, I would rather cache those results internally within Datasette after they are first calculated than continue to support them in the datasette inspect command - to keep things simpler.