wider margin for collapse timing + use slowdown in test_concurrent_collapse_fails

first commit is motivated by http://172.19.49.85/view/arakoon-1.7/job/arakoon-1.7-system-tests-slow-LEFT/lastCompletedBuild/testReport/arakoon_system_tests.server.left/test_collapse_slowdown/test_collapse_slowdown/
second commit is motivated by http://172.19.49.85/view/arakoon-git/job/arakoon-git-system-tests-slow-LEFT/482/testReport/arakoon_system_tests.server.left/test_concurrent_collapse_fails/test_concurrent_collapse_fails/