Happiness_Upload now takes in full_peers

The PeerSelector now passes the set of full peers to Happiness_Upload.
Also removes the case in which no servermap is supplied from
generate_mappings because the case is redundant.
Fixes construction of peer priorities

Fixes an error in _distribute_homeless_shares where the loop
was trying to assign a priority to full_peers when full_peers
are not present in the priority dictionary. If the loop encounters
a peer that is full, it just skips over that peer.