Add support for INSERT, DELETE and UPDATE.
This is a major rewrite that might cause some destabilization in existing functionality.
Add new column option "key" to indicate primary key columns (needed for UPDATE and DELETE).
Add new table option "readonly" to forbid data modifying statements.  Queries against such tables will use READ ONLY transactions.
Improved transaction handling for data modifying statements.  COMMIT, ROLLBACK and SAVEPOINT will work as expected.