- provides ability to cache foreign slices

- provides ability to cache foreign slices
  only LOCAL nodes are known to foreign slices, with their local id as expected

- RefreshPeer logic currently is
  *) invoke GetNodes and cache into ForeignNodes
  *) invoke ForeignNodes, not cached for now, used in next step
  *) invoke GetSlices, then caches into ForeignSlices,
     only local nodes are considered, with their alien node_ids transcoded
     to local node_ids

- gets rid of the foreign_id columns in peer_node and peer_slice,
  that turned out useless

=== a few pending issues

- GetSlivers not yet updated accordingly
  it still works on non-federated plcs

- site_id and creator_person_id in the slices table
  are not declared 'NOT NULL' anymore.
  as far as GetSlices is concerned these entries are still non null,
  as per the WHERE stuff added on view_slices

- refreshing the db to update the slice x node association is done
  in a clear-all/add-all fashion that clearly sucks.

- the caching logic is currently in Peers.py which is clearly wrong
  there's a need for refactoring these 2 types of cached objects
  to provide more genericity