Case: Called from ReaderThread on complete message or fragments.
The given buf may be entire message or a fragment.
The ReaderThread finds the ReadBufferManager instance either in
in a fragment map (when collecting - GIOP 1.2 phase 1) or
in an active server requests map (when streaming - GIOP 1.2 phase 2).
As a model for implementation see IIOPInputStream's
constructor of the same name. There are going to be some variations.