Drush DB tweak/Fix

Some websites (currently IPPOG) faced an issue where DB-Dumping was not possible with the following error:

> mysqldump: Error 2020: Got packet bigger than 'max_allowed_packet' when dumping tablewebprofilerat row: 1195

This is because we were using the default max_allowed_packets. This MR proposes to use 512MB by default.

As a "bonus", database-restore now won't immediately drop tables, avoiding any possible manual execution by mistake which could have critical failures.