Paxos requires all nodes for CAS

In C* 2.1,

int requiredParticipants = participants + 1  / 2; // See CASSANDRA-833

Will always return participants because of operator precedence. I am not sure just adding parentheses will fix the problem, though, as the original code differentiated between pending and natural endpoints.

 int requiredParticipants = pendingEndpoints.size() + 1 + naturalEndpoints.size() / 2; // See CASSANDRA-833