When trying to exit with a hard exit (exit!), runtime teardown is
performed by the shutdown hook thread added in Main.internalRun(), which
is run on a different thread.

This poses a problem when using ruby-debug, as it grabs a lock while
processing debug events [1], and thus if you try to exit! while trying to
process debug events (for instance if you're debugging interactively
with pry and you want to exit), you'll get a deadlock looking something
like this:

```
"Thread-1" - Thread t@11
   java.lang.Thread.State: BLOCKED
  at org.jruby.debug.DebugEventHook.eventHandler(DebugEventHook.java:91)
  - waiting to lock <3b88136f> (a org.jruby.debug.DebugEventHook) owned by "main" t@1
  at org.jruby.runtime.EventHook.event(EventHook.java:30)
  at org.jruby.Ruby.callEventHooks(Ruby.java:3179)
  at org.jruby.runtime.ThreadContext.trace(ThreadContext.java:652)
  (...)
  at org.jruby.RubyProc.call(RubyProc.java:249)
  at org.jruby.Ruby.tearDown(Ruby.java:3281)
  at org.jruby.Ruby.tearDown(Ruby.java:3254)
  at org.jruby.Main$1.run(Main.java:293)

   Locked ownable synchronizers:
  - None

"main" - Thread t@1
   java.lang.Thread.State: WAITING
  at java.lang.Object.wait(Native Method)
  - waiting on <5579bb86> (a org.jruby.Main$1)
  at java.lang.Thread.join(Thread.java:1249)
  at java.lang.Thread.join(Thread.java:1323)
  at java.lang.ApplicationShutdownHooks.runHooks(ApplicationShutdownHooks.java:106)
  at java.lang.ApplicationShutdownHooks$1.run(ApplicationShutdownHooks.java:46)
  at java.lang.Shutdown.runHooks(Shutdown.java:123)
  at java.lang.Shutdown.sequence(Shutdown.java:167)
  at java.lang.Shutdown.exit(Shutdown.java:212)
  - locked <39d0fc85> (a java.lang.Class)
  at java.lang.Runtime.exit(Runtime.java:109)
  at java.lang.System.exit(System.java:971)
  at org.jruby.RubyKernel.exit(RubyKernel.java:708)
  at org.jruby.RubyKernel.exit_bang(RubyKernel.java:683)
  (...)
  at org.jruby.debug.DebugEventHook.eventHandler(DebugEventHook.java:97)
  - locked <3b88136f> (a org.jruby.debug.DebugEventHook)
  at org.jruby.runtime.EventHook.event(EventHook.java:30)
  at org.jruby.Ruby.callEventHooks(Ruby.java:3179)
  at org.jruby.runtime.ThreadContext.trace(ThreadContext.java:652)
  (...)
  at org.jruby.Main.main(Main.java:204)
```

To fix this, we disable event hooks during teardown, so that code events
during this period are not sent to the debugger.

[1]: https://github.com/ruby-debug/ruby-debug/blob/master/src/org/jruby/debug/DebugEventHook.java#L91-L101