Allow foreign key constraints to be disabled in batches

Our code currently implicitly wraps batches of statements into BEGIN + COMMIT or BEGIN + ROLLBACK. Problem is, in order to temporarily turn off foreign key contraints users might want to send a PRAGMA FOREIGN_KEYS=off statement as a first statement in the batch. Unfortunately, this pragma is not effective inside a transaction: https://www.sqlite.org/foreignkeys.html#fk_enable

A simple workaround would be to check if the first statement happens to be a pragma and execute it outside of the transaction scope. Another simple workaround would be to add a special interface that allows you to say "please run this batch without foreign keys", and then we'd send a PRAGMA FOREIGN_KEYS=off ourselves, right before the BEGIN+COMMIT block.

Start by locating the batch execution path that currently wraps statements in BEGIN and COMMIT or ROLLBACK. Read the SQLite foreign-key pragma documentation linked in the issue, then determine which proposed interface or transaction-boundary behavior should be implemented; done means batches can disable foreign-key enforcement before the transaction begins.