MAJOR: connection: split the send call into connection and stream interface

MAJOR: connection: split the send call into connection and stream interface

Similar to what was done on the receive path, the data layer now provides
only an snd_buf() callback that is iterated over by the stream interface's
si_conn_send_loop() function.

The data layer now has no knowledge about channels nor stream interfaces.

The splice() code still need to be ported as it currently is disabled.