Staging: dst: export node.

This patch introduces remote (export) node machinery: initialization
address/port (and other socket parameters), export block device (can be
another DST storage for example or local device like /dev/sda1), local
IO processing engine (BIO state machines, receiving/submitting logic).
Network management for the export node like accepting new client, scheduling
its command processing thread, receiving/sending IO requests, all are placed
here.

Signed-off-by: Evgeniy Polyakov <zbr@ioremap.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>