Improve SuspendController code

From Brians's comment at https://github.com/wildfly/wildfly-core/pull/3336#pullrequestreview-125435804:

Reviewing this I see a few issues in SuspendController. 1) The Timer allocated at L98 is pointless unless timeoutMillis > 0. 2) The timeout() method can be private (same with activityPaused()). 3) The logging at L74-L78 is incorrect if timeoutMillis == 0. The "suspendingServerWithNoTimeout" message is not correct in that case as 'no timeout' doesn't mean '0 ms'.