testrunner data_helper memcached loader crashes while running test against physical machines

in testrunner memcached helper we try to remember all the keys inserted to the memcached for verification purposes.
this does not make sense when the test ends up inserting more than 10-20 million items
this causes memory overflow and lot of other problems in the test case.

instead we should simply have a formula where we can use to construct which keys were inserted and etc.