[Hello-Java.Base] MonoVM + peer collection (#1319)

Context: a5b229d

Now that MonoVM can be used on the desktop (a5b229d), let's show it
in action!

Update `samples/Hello-Java.Base` to call `GC.Collect()` and
`GC.WaitForPendingFinalizers()` to help cause the `MyJLO` instance to
be collected before app exit.

Which is fine, but what we also need are *GREF logs*.  Unfortunately,
we can't use `JreRuntimeOptions.JniGlobalReferenceWriter` for that
purpose, because `libjava-interop.dylib` is native code and we want
both managed and native code to write GREF logs to the same place.

Instead, set the `JAVA_INTEROP_GREF_LOG` environment variable to a
file path:

	% JAVA_INTEROP_GREF_LOG=g.txt  ./samples/Hello-Java.Base/bin/Release/osx-x64/publish/Hello-Java.Base
	MonoVM support enabled
	# jonp: LoadJvmLibrary(/Library/Java/JavaVirtualMachines/microsoft-17.jdk/Contents/Home/lib/libjli.dylib)=1164832560
	# jonp: JNI_CreateJavaVM=4554903296; JNI_GetCreatedJavaVMs=4554903376
	# jonp: executing JNI_CreateJavaVM=10f7e4f00
	# jonp: r=0 javavm=111647690 jnienv=7fbfb7afbaa8
	WARNING in native method: JNI call made without checking exceptions when required to from CallStaticObjectMethodV
	binding? net.dot.jni.sample.MyJLO@575bab75
	WARNING in native method: JNI call made without checking exceptions when required to from CallVoidMethodV

Then read `g.txt`.  Selected output includes:

	+g+ grefc 5 gwrefc 0 obj-handle 0x7fbfb6104920/L -> new-handle 0x7fbfb57274c0/G from thread '(null)'(4)
	   at Java.Interop.MonoRuntimeObjectReferenceManager.CreateGlobalReference(JniObjectReference reference)
	   at Java.Interop.JniObjectReference.NewGlobalRef()
	   at Java.Interop.JniRuntime.JniValueManager.ConstructPeer(IJavaPeerable peer, JniObjectReference& reference, JniObjectReferenceOptions options)
	   at Java.Interop.JavaObject.Construct(JniObjectReference& reference, JniObjectReferenceOptions options)
	   at Java.Lang.Object..ctor()
	   at Hello.MyJLO..ctor()
	   at Hello.App.<>c.<CreateJLO>b__3_0()
	   at System.Threading.Thread.StartCallback()

	Created PeerReference=0x7fbfb57274c0/G IdentityHashCode=0x575bab75 Instance=0xef12a08e Instance.Type=Hello.MyJLO, Java.Type=net/dot/jni/sample/MyJLO

	*take_weak obj=0x10fca0478; handle=0x7fbfb57274c0
	+w+ grefc 7 gwrefc 1 obj-handle 0x7fbfb57274c0/G -> new-handle 0x7fbf9570a801/W from thread '(null)'(1)
	take_weak_global_ref_jni
	-g- grefc 6 gwrefc 1 handle 0x7fbfb57274c0/G from thread '(null)'(1)
	take_weak_global_ref_jni
	*try_take_global obj=0x10fca0478 -> wref=0x7fbf9570a801 handle=0x0
	-w- grefc 6 gwrefc 0 handle 0x7fbf9570a801/W from thread '(null)'(1)
	take_global_ref_jni
	Finalizing PeerReference=0x0/G IdentityHashCode=0x575bab75 Instance=0xef12a08e Instance.Type=Hello.MyJLO

Which shows the creation of JNI handle 0x7fbfb57274c0/G, which is
associated with `Hello.MyJLO`, and then we go through the GC process:

  * Create a JNI Weak Global Reference 0x7fbf9570a801/W (`+w+ …`)
  * Delete the JNI Global Reference 0x7fbfb57274c0/G (`-g- …`)
  * Perform a Java-side GC (not mentioned)
  * Try to create a JNI Global Reference from the Weak Global Reference
    (`*try_take_global …`)
  * Delete the JNI Weak Global Reference (`-w- …`)
  * Finalize the `Hello.MyJLO` instance (`Finalizing PeerReference=0x0/G …`)

The `Instance=` value can be used to correlate instances; it's the
value of `RuntimeHelpers.GetHashCode()`.

Author: jonpryor

samples/Hello-Java.Base/Program.cs

@@ -38,13 +38,7 @@ public static void Main (string[] args)
 				  v => reportTiming = v != null },
 				{ "v|verbosity:",
 				  $"Set console log verbosity to {{LEVEL}}.  Default is 0.",
-				  (int? v) => {
-				    verbosity = v.HasValue ? v.Value : verbosity + 1;
-				    if (verbosity > 0) {
-				      logger    = CreateConsoleLogger ();
-					}
-				  }
-				},
+				  (int? v) => verbosity = v.HasValue ? v.Value : verbosity + 1 },
 				{ "h|help",
 				  "Show this message and exit.",
 				  v => showHelp = v != null },
@@ -54,6 +48,9 @@ public static void Main (string[] args)
 				options.WriteOptionDescriptions (Console.Out);
 				return;
 			}
+			if (verbosity > 0) {
+				logger = CreateConsoleLogger ();
+			}
 			var builder = new JreRuntimeOptions () {
 				JniAddNativeMethodRegistrationAttributePresent  = true,
 				JvmLibraryPath                                  = jvmPath ?? global::Java.InteropTests.TestJVM.GetJvmLibraryPath (logger),
@@ -79,6 +76,11 @@ public static void Main (string[] args)
 			}
 
 			CreateJLO ();
+
+			GC.Collect ();
+			GC.Collect ();
+			GC.WaitForPendingFinalizers ();
+			GC.WaitForPendingFinalizers ();
 		}
 
 		static Action<TraceLevel, string> CreateConsoleLogger ()