Fix clang warnings (#449)

* Remove TryCatch compiler warning

```
Release/obj.target/appmetrics/geni/appmetrics.cpp:396:18: warning: 'TryCatch' is deprecated [-Wdeprecated-declarations]
        TryCatch try_catch;
                 ^
```

* Fix CallAsFunction compiler warning

```
Release/obj.target/appmetrics/geni/appmetrics.cpp:597:93: warning: 'CallAsFunction' is deprecated [-Wdeprecated-declarations]
    Local<Value> m = module->Get(Nan::New<String>("require").ToLocalChecked())->ToObject()->CallAsFunction(Nan::GetCurrentContext()->Global(), 1, args);
                                                                                            ^
/Users/gib/.cache/node-gyp/6.10.3/include/node/v8.h:2984:30: note: 'CallAsFunction' has been explicitly marked deprecated here
                Local<Value> CallAsFunction(Local<Value> recv, int argc,
                             ^
```

* Fix other compiler warnings

Author: gibfahn

src/appmetrics.cpp

@@ -393,7 +393,9 @@ static void emitMessage(uv_async_t *handle, int status) {
     uv_mutex_unlock(messageListMutex);
 
     while(currentMessage != NULL ) {
+      
         Nan::TryCatch try_catch;
+
         const unsigned argc = 2;
         Local<Value> argv[argc];
         const char * source = (*currentMessage->source).c_str();

src/plugins/node/gc/nodegcplugin.cpp

@@ -128,11 +128,11 @@ static unsigned long long GetRealTime() {
 }
 #endif
 
-void beforeGC(GCType type, GCCallbackFlags flags) {
+void beforeGC(v8::Isolate *isolate, GCType type, GCCallbackFlags flags) {
 	plugin::timingOK = GetSteadyTime(&plugin::gcSteadyStart);
 }
 
-void afterGC(GCType type, GCCallbackFlags flags) {
+void afterGC(v8::Isolate *isolate, GCType type, GCCallbackFlags flags) {
 	unsigned long long gcRealEnd;
 
 	// GC pause time
@@ -201,12 +201,12 @@ extern "C" {
 
 	NODEGCPLUGIN_DECL int ibmras_monitoring_plugin_start() {
 		plugin::api.logMessage(fine, "[gc_node] Starting");
-	
-		V8::AddGCPrologueCallback(beforeGC);
-		V8::AddGCEpilogueCallback(afterGC);
+
+		v8::Isolate::GetCurrent()->AddGCPrologueCallback(beforeGC);
+		v8::Isolate::GetCurrent()->AddGCEpilogueCallback(afterGC);
 		return 0;
 	}
-	
+
 	NODEGCPLUGIN_DECL int ibmras_monitoring_plugin_stop() {
 		plugin::api.logMessage(fine, "[gc_node] Stopping");
 		// TODO Unhook GC hooks...

src/plugins/node/loop/nodeloopplugin.cpp

@@ -51,10 +51,6 @@ static char* NewCString(const std::string& s) {
 	return result;
 }
 
-static void cleanupHandle(uv_handle_t *handle) {
-	delete handle;
-}
-
 uv_check_t check_handle;
 int32_t min = 9999;
 int32_t max = 0;