Currently criu sends data to the page server, but it doesn't get any
feedback, so it can't be sure that all data have been accepted.

This patch adds a flush command, which requires an answer from the page
server. This command is sent before disconnecting.
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>