cgen: duplicate callback methods

If a single callback method is mapped to multiple events, this method is generated N number of times. E.g.:

<obj-entity ...>
...
<pre-persist method-name="preCommit"/>
<pre-update method-name="preCommit"/>
</obj-entity>

public abstract class _X extends CayenneDataObject

{ ... protected abstract void preCommit(); protected abstract void preCommit(); }

I tried this on 3.1, but I am pretty sure this is broken in 3.0 as well.