(*) slice atttribute types get cached

(*) slice atttribute types get cached
(*) TestPeers to create custom slice attribute types and slice attributes
(*) new method GetPeerData to allow RefreshPeer to perform
    a single xmlrpc request (overall improvement : x 3 on small deployments)
(*) Cache created with a peer_id rather than with a peer,
    so as to remove a useless dependency (arg check done by RefreshPeer)
(*) caching slice attributes will require another method,
    equality cannot be based on a single column as for other types
    of cached data; ongoing