[CRIU] Lazy-restore design discussion - round 3

[Adrian Reber]

The new summary:

 * On the source system there will be process listening on a network
   socket. In the first implementation it will use a checkpoint
   directory as the basis for the UFFD pages and in a later version
   we will add the possibility to transfer the pages directly from the
   checkpointed process.

 * The transport protocol between the source system and the UFFD daemon
   on the destination will be page-server based (something like Mike's patch)

 * The UFFD daemon will be able to handle multiple restore requests
   (also Mike's patch "lazy-pages: handle multiple processes")
   to restore a whole process tree.

 * For each restored process tree a new UFFD daemon is started.

 * The UFFD daemon is either started manually on the command-line
   or automatically started by the restore process.

 * The UFFD daemon reads all required information (besides PID and UFFD)
   from the checkpoint directory.

 * The UFFD daemon is the instance which decides which pages are pushed
   when via UFFD into the restored process.

Right now I am trying to find the minimal required points to get this
implemented. Once this is done we can continue from there and start more
discussions.

		Adrian