Get the health score for a peer.
    Health is: (number of responses received / number of requests sent)

def atlas_peer_get_health( peer_hostport, peer_table=None ):
    """
    Get the health score for a peer.
    Health is: (number of responses received / number of requests sent) 
    """
    with AtlasPeerTableLocked(peer_table) as ptbl:
        # availability score: number of responses / number of requests
        num_responses = 0
        num_requests = 0
        if ptbl.has_key(peer_hostport):
            for (t, r) in ptbl[peer_hostport]['time']:
                num_requests += 1
                if r:
                    num_responses += 1

        availability_score = 0.0
        if num_requests > 0:
            availability_score = float(num_responses) / float(num_requests)

    return availability_score