WL#3050: Change binlog to be engine

As discussed during the Seattle Dev Managers' meeting
on 2006-01-05, the task is to change the binlog to be
a [pluggable] storage engine.

Additional information for this task (from Elliot Murphy):
     "One of the long term goals for this is that we would be able to 
switch to different storage engines (like NDB) for holding the binlog 
at some point in the future. Making interaction with the binlog occur 
through a normal engine interface is one step towards that.
     Of course, selecting from binlog would also be nice."
-- Trudy Pelzer, 2006-01-19