Fix race condition in Time.java

Some of my test runs were occasionally failing with a NullPointerException on `backtype.storm.utils.Time.java:64`.

After a bit of investigation, it seems there's a race condition here; if we disable simulating mode while a thread is currently sleeping, then when it wakes up it won't re-check if it's still in "simulating" mode, it'll try to remove the sleep time, and get the NPE.

Refs [STORM-260](https://issues.apache.org/jira/browse/STORM-260).