%% A message_cb is invoked whenever a message is sent or received, or
%% to provide acknowledgement of a completed send or discarded
%% request. Ignoring possible extra arguments, calls are of the
%% following form.
%%
%% cb(recv, Msg)          Receive a message into diameter?
%% cb(send, Msg)          Send a message on the socket?
%% cb(ack,  Msg)          Acknowledgement of a completed send.
%% cb(ack,  false)        Acknowledgement of a discarded request.
%%
%% Msg will be binary() in a recv callback, but can be a
%% diameter_packet record in a send/ack callback if a recv/send
%% callback returns a record. Callbacks return a list of the following
%% form.
%%
%%   [boolean() | send | recv | binary() | #diameter_packet{}]
%%
%% The atoms are meaningless by themselves, but say whether subsequent
%% messages are to be sent or received. A boolean says whether or not
%% to continue reading on the socket. Messages can be received even
%% after false is returned if these arrived in the same packet. A
%% leading recv or send is implicit on the corresponding callbacks. A
%% new callback can be returned as the tail of a returned list: any
%% value not of the aforementioned list type is interpreted as a
%% callback.