Several tests are flaky, with futures timing out early

Following the merging of a large patch chain, it was noticed on our internal CI that several tests had become flaky, with a similar pattern in the failures: the tests fail early when a future times out. Often, this occurs when a test cluster is being spun up and one of the offer futures times out. This has been observed in the following tests:

• MesosContainerizerSlaveRecoveryTest.CGROUPS_ROOT_PidNamespaceForward
• MesosContainerizerSlaveRecoveryTest.CGROUPS_ROOT_PidNamespaceBackward
• ZooKeeperStateTest.FetchAndStoreAndStoreAndFetch
• RoleTest.ImplicitRoleRegister
• SlaveRecoveryTest/0.MultipleFrameworks
• SlaveRecoveryTest/0.ReconcileShutdownFramework
• SlaveTest.ContainerizerUsageFailure
• MesosSchedulerDriverTest.ExplicitAcknowledgements
• SlaveRecoveryTest/0.ReconnectHTTPExecutor (MESOS-6164)
• ResourceOffersTest.ResourcesGetReofferedAfterTaskInfoError (MESOS-6165)
• SlaveTest.CommandTaskWithKillPolicy (MESOS-6166)

See the linked JIRAs noted above for individual tickets addressing a couple of these.