[#2744] Fix flakey HashedWheelTimerTest.testExecutionOnTime()

Motivation:

The calculation of the max wait time for HashedWheelTimerTest.testExecutionOnTime() was wrong and so the test sometimes failed.

Modifications:

Fix the max wait time.

Result:

No more test-failures
ref #2649
Merge branch '4.1' of https://github.com/netty/netty into 4.1