dtest for many UPDATE batches, low contention fails on trunk

paxos_tests.py:TestPaxos.contention_test_multi_iterations fails consistently on trunk (cassci history). The test works by creating 8 workers, each of which increments an integer 100 times using UPDATE. Based on the test failures, it looks like 2 or 3 of the updates are consistently dropped. Other dtests that don't run as many iterations but have more contention succeed.

I'm assigning you, slebresne, because you wrote the test and were the last person to modify it, but feel free to reassign. Thanks!