rst: Introduce an engine to allocate memory on restore

On restore we need differetn types of memory allocation.
Here's an engine that tries to generalize them all. The
main difference is in how the buffer with objects is being
grown up.

There are 3 types of memory allocations:

1. shared memory -- objects, that will be used by all criu
   children, but will not reach the restorer
2. shared remapable -- the same, but restorer would need
   access to them, i.e. -- buffer with objects will get
   remapped into restorer
3. private -- the same, but allocatedby each task for itself

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