change names of events to V2 from default

according to this post
http://dev.mysql.com/doc/internals/en/row-based-replication.html#rows-event
events are marked as version 0,1,2 (current) - the code had _V1 and no
version at all - I think it would b easier and morefuture compatible to
use V2 since it seems V3 is unavoidable.