mem: Introduce page server

The page server is a process, that is about to get pages over
the network and put them into pagemap- + pages- images. Right
now what it does is simply get the data and puts it into the
image files. When we have dirty set tracking in the kernel the
page server will have to collect "page changes" and properly
integrate them into images.

Running crtools with page server is like this:

dst_node# crtools page-server --port <port> -D dump/ ...
src_node# crtools dump -t <pid> --page-server --address <dst_node> --port <port> -D dump/ ...

After this images from dst_node/dump/ and src_node/dump/ should
be put into one place and tasks can be restored out of it.

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