locator: Speed up simple_strategy::calculate_natural_endpoint
locator: Speed up simple_strategy::calculate_natural_endpoint

If the number of nodes in the cluster is smaller than the desired
replication factor we should return the loop when endpoints already
contains all the nodes in the cluster because no more nodes could be
added to endpoints lists

Refs #10337
Refs #10817
Refs #10836
Refs #10837

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla