Randomize memcached checks

The point is to make sure we don't miss detecting a dead memcached instance for too long.

In our M3 setup, we distribute over memcached instances with a key hashing algorithm, so sending enough random keys should ensure we talk to all instances.

Users who are not interested in this feature can set the number of random checks to 0 (which defaults to using always the same key "ping")
Might be a good idea to have a test that makes sure the sequence is `n` long.