Stop abusing System.gc() on Commons Daemon jsvc shutdown

The shutdown sequence invokes System.gc() several times. I'm not sure why this would be necessary, especially considering that System.gc() is only a hint and the JVM will not necessarily do anything when this method is invoked. Especially if -XX:+DisableExplicitGC is used, which is fairly common practice in production environments.

In our case, we use rather huge heaps (over 100 GB) so shutdown can take forever, depending on how much uncollected garbage there is in the JVM heap. We've always used -XX:+DisableExplicitGC so this was never apparent, however we decided to stop using -XX:+DisableExplicitGC after careful code review showed none of our apps or supporting libraries are using System.gc() and sometimes we want to trigger GC manually from monitoring tools (Jconsole, VisualVM, etc.) - but we found the hard way that Commons Daemon / jsvc is using System.gc() on shutdown, preventing the speedy shutdown we were used to.

Re-introducing XX:+DisableExplicitGC fixes the problem, and shutdown is back to normal, takes less than 8 seconds on our hardware.