Truncate all tables in a single command, fix #61.

The truncate command is only sent to PostgreSQL when we didn't just
CREATE TABLE before. Some refactoring would be necessary to fit the
TRUNCATE command within the same transaction as the CREATE TABLE
command, for PostgreSQL performances.

This patch has been testing with MySQL and SQLite sources, the trick is
that to be able to test it, it's needed to first make a full
import (creating the target tables), so the test are not modified yet.