Upgrades: check db consistency if possible

I attach to this issue a modified script from http://forge.mysql.com/tools/tool.php?id=11 that checks a database's contents for consistency with other database's schema.

After importing it into the database with data, you do

call p_check_fk_constraint_violations('database-with-schema', '%')

and it returns whether the database with data fulfils database-with-schema's constraints.

Aaand the gist of this issue is to hook this to our upgrade scripts - if the MySQL user has CREATE DATABASE permission, make a database with the schema you're upgrading from (i.e., 2.1) and compare the contents of their database with the above call, before proceeding with the upgrade.