Add manual exit at the end of FUSE life cycle

[jiacheliu3]

What changes are proposed in this pull request?

Added a manual double-safety System.exit() at the end of FUSE lifecycle, to be absolutely sure we exit the JVM and terminate all non-daemon & daemon threads.

Before this change, after we alluxio-fuse umount or kill (not -9), the FUSE process may fail to quit. One reason is a non-daemon thread dangling around:

# This is a non daemon thread in AlluxioEtcdClient
"vert.x-eventloop-thread-0" #21 prio=5 os_prio=31 cpu=482.23ms elapsed=722.11s tid=0x00007fde79251800 nid=0x8c03 runnable  [0x00007000102ae000]
   java.lang.Thread.State: RUNNABLE
   at sun.nio.ch.KQueue.poll(java.base@11.0.11/Native Method)
   at sun.nio.ch.KQueueSelectorImpl.doSelect(java.base@11.0.11/KQueueSelectorImpl.java:122)
   at sun.nio.ch.SelectorImpl.lockAndDoSelect(java.base@11.0.11/SelectorImpl.java:124)
   - locked <0x00000007c0603938> (a io.netty.channel.nio.SelectedSelectionKeySet)
   - locked <0x00000007c06038d8> (a sun.nio.ch.KQueueSelectorImpl)
   at sun.nio.ch.SelectorImpl.select(java.base@11.0.11/SelectorImpl.java:136)
   at io.netty.channel.nio.SelectedSelectionKeySetSelector.select(SelectedSelectionKeySetSelector.java:62)
   at io.netty.channel.nio.NioEventLoop.select(NioEventLoop.java:883)
   at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:526)
   at io.netty.util.concurrent.SingleThreadEventExecutor$4.run(SingleThreadEventExecutor.java:997)
   at io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74)
   at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)
   at java.lang.Thread.run(java.base@11.0.11/Thread.java:829)

The JVM shutdownHook executes after the LAST non-daemon thread dies or when System.exit() triggers. So if some libraries we use or some thread pools of ours accidentally introduce non-daemon threads, System.exit() is our last resort to trigger shutdownHook.

FUSE does handle signals and exit, but there are some corner cases which don't seem to trigger that successfully. So this PR serves as the last resort.

alluxio/dora/integration/fuse/src/main/java/alluxio/fuse/FuseSignalHandler.java

Line 52 in 527ffe8

System.exit(0);

Why are the changes needed?

See above

Does this PR introduce any user facing changes?

No

[jiacheliu3]

alluxio-bot, merge this please