fixed a bug in message handling for code the change notify code

fixed a bug in message handling for code the change notify code

The change notify code registered a separate message handler for each
tree connect. This registration uses the global messaging context.

The messaging code would consider a 2nd registration for the same
messaging type as being an 'update' of the handler, rather than a new
handler. It also would only call the first handler in the linked list
for a given message type when dispatching messages.

This patch changes the messaging code to allow for multiple
registrations of the same message type, and allow for multiple calls
to different messaging handler for one incoming message.

This fixes the problem with the test_notify_tcon() test that I
recently committed to the S4 smbtorture