Add NativeMigrator that uses Redis MIGRATE command

Should work mostly the same as before (except changed copy_set and copy_list a bit, added specs that would check if the data is copied in exactly the same form and these two methods were failing).

Still want to do before any of this could be merged:
* performance testing
* some live migration testing (can't rely too much on specs here since MockRedis doesn't support MIGRATE command)
* more error handling (the native command allows for easy retrying)
* any comments/feedback from You on what could be improved/fixed