service: Implement pre-dump request support
service: Implement pre-dump request support

This works as multi-req thing -- caller issues the
pre-dump request, criu serves it and sends result back.
Then service waits for the next request on the same
session, client doesn't have to re-connect back.

Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@parallels.com>

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