Problem: Replication fails when master is mysql-5.1-wl2325-5.0-drop6 and
slave is mysql-5.1-new-rpl. The reason is that, in
mysql-5.1-wl2325-5.0-drop6, the event type id's were different than in
mysql-5.1-new-rpl.
Fix (in mysql-5.1-new-rpl):
 (1) detect that the server that generated the events uses the old
format, by checking the server version of the format_description_log_event
This patch recognizes mysql-5.1-wl2325-5.0-drop6p13-alpha,
mysql-5.1-wl2325-5.0-drop6, mysql-5.1-wl2325-5.0, mysql-5.1-wl2325-no-dd.
 (2) if the generating server is old, map old event types to new event
types using a permutation array.

I've also added a test case which reads binlogs for four different
versions.