Minor memory leak in C++ broker

[Ted Ross]

There's a bug in QueueBindings.cpp (QueueBindings::unbind) where binding 
records are not deleted when bindings/queues are removed.  Except for 
the memory leak, this is not a problem for operation.  It is, however, a 
problem for management as I now see orphaned bindings accumulating.

As far as I can tell, the only reason for this data structure to exist 
is to unbind all bindings associated with a particular queue.  Since 
bindings are a "manageable" entity (i.e. they have statistics and other 
information associated with them), there might be a better way to 
structure them such that they are more closely tied to the exchanges 
that use them.

Unless anybody has a different idea, I will propose a patch to 
restructure this (and fix the memory leak).

-Ted