Idle sessions are no longer being closed even though TimeoutLifecycleManagement is configured properly

Sometimes the timer thread which is initialized by the TimeoutLifecycleManager crashes. This leads to zeppelin not clearing any timedout sessions until a server restart is performed.

The underlying mechanism uses a java Timer (inside TimeoutLifecycleManager.java). We are not sure what had caused the TimerThread thread to crash, since it swallows exceptions/errors. However, a jstack comparison between the server and another zeppelin server which was working well had shown that there is a missing TimerThread thread in the problematic server (1 TimerThread thread vs. 2 TimerThreads). After restarting the zeppelin server, jstack started showing 2 TimerThread threads in its output.

Running on jdk 1.8.0_151, ubuntu 14.04, EC2.