Make zbigMapReduce.js more resilient on slow machines in 3.6

zbigMapReduce.js can fail on slow machines if enough balancer triggered migrations complete during a batch write to prevent the write from making progress for 5 attempts. Previously we mitigated this by increasing the interval after a successful balancer round in this test (in SERVER-32692), but the failure is still happening on some variants in 3.6. To make the test more reliable we should further increase the interval just in 3.6.