Revision 397980 changes MessageId to ClientMessageId

[David W. Van Couvering]

Hi, all.  It turns out I needed iapi/reference/MessageId.java on the 
client side, so I moved it to shared/common/reference.  Then, to avoid 
class name confusion, I renamed client/am/MessageId to 
client/am/ClientMessageId.  I just checked this change in (passes 
derbynetclientmats cleanly).  This may cause some conflicts in your sandbox.

David