Disable Foreign Key snapshot generation if Database supports Foreign Key...

It takes to much time to drop all database objects  from a very big MySQL Database. 
As long as MySQL supports a Disable Foreign Key command a lot of "DROP FOREIGN KEY ..." queries can be avoided by excluding ForeignKey from a set of DatabaseSnapshot objects. 

All other keys removal (Primary, Unique, Index) can be skipped as well because they all should be removed together with containing tables.