zyre_peers - return a zlist, not a zhash, for safety

Specifically, if the user is tempted to access the elements in the
returned hash (as I was) then they will open themselves to unpleasant
race conditions with the node actor. Simply returning the peer ids in a
list makes it impossible for this to occur.
Merge pull request #251 from twhittock/peers-safety

zyre_peers - return a zlist, not a zhash, for safety