crc32 hash value conflict in add function

[andyxning]

i think we should take crc32 hash value conflict in add function, as shown in

// need c.Lock() before calling
func (c *Consistent) add(elt string) {
    for i := 0; i < c.NumberOfReplicas; i++ {
    c.circle[c.hashKey(c.eltKey(elt, i))] = elt
    }
    c.members[elt] = true
    c.updateSortedHashes()
    c.count++
}

the c.hashKey(c.eltKey(elt, i)) may return a conflict, i.e., a duplicate crc32 hash value and in such a situation we can not guarantee that we can have specified NumberOfReplicas for each node.

For more info, an example can be get in carbon

def add_node(self, node):
    self.nodes.add(node)
    for i in range(self.replica_count):
      replica_key = "%s:%d" % (node, i)
      position = self.compute_ring_position(replica_key)
      while position in [r[0] for r in self.ring]:
        position = position + 1
      entry = (position, node)
      bisect.insort(self.ring, entry)