Input processing used to keep all data in one single input buffer, which
makes it hard to handle nested events as well as reading directly from the
socket into the application buffer without an extra copy.
Create a new iscsi_in_pdu structure where we store the header, and any data
for the recevied pdu and store them in a proper input queue.
Change the signature for all processing functions to tahe a iscsi_in_pdu
structure for the received pdu instead of just a pointer to a buffer.
