cleanup document class migration command, add missing generate proxies command

DocumentMigrateClassCommand was not using the DocumentClassMapper but mimicking the default DocumentClassMapper behaviour (with an inconsistency: storing all parent classes instead of all *mapped* parent classes).

I also copied the generate proxies command from doctrine orm. This command is not needed in the symfony bundle as its done in a cache warming listener - the orm is doing the same, not exposing the proxy command to the symfony console. This command is essential to use doctrine phpcr-odm standalone.