Fix deprecated NAN async callback usage

[joelpurra]

Deprecation warnings (since Node.js v10):

• Nan::MakeCallback(...)
• Call(...) on Nan::Callback

Should either use/pass in Nan::AsyncResource, or switch to Nan::AsyncWorker (which builds upon Nan::AsyncResource).

The effect on asynchronous DNS requests should be assessed, and possibly improved. This change may help with cleanup of both finished and unfinished requests.

See

• https://travis-ci.org/github/getdnsapi/getdns-node/jobs/769298461#L309-L393
• https://github.com/nodejs/nan
• https://github.com/nodejs/nan/blob/master/doc/node_misc.md#nanmakecallback
• https://github.com/nodejs/nan/blob/master/doc/node_misc.md#api_nan_asyncresource
• https://github.com/nodejs/nan/blob/master/doc/asyncworker.md#api_nan_async_worker

../src/GNContext.cpp: In static member function ‘static void GNContext::Callback(getdns_context*, getdns_callback_type_t, getdns_dict*, void*, getdns_transaction_t)’:
../src/GNContext.cpp:832:69: warning: ‘v8::Local<v8::Value> Nan::Callback::Call(v8::Local<v8::Object>, int, v8::Local<v8::Value>*) const’ is deprecated [-Wdeprecated-declarations]
  832 |     data->callback->Call(Nan::GetCurrentContext()->Global(), 3, argv);
      |                                                                     ^
In file included from ../src/GNContext.h:32,
                 from ../src/GNContext.cpp:28:
../node_modules/nan/nan.h:1720:3: note: declared here
 1720 |   Call(v8::Local<v8::Object> target
      |   ^~~~
../src/GNContext.cpp: In static member function ‘static Nan::NAN_METHOD_RETURN_TYPE GNContext::Lookup(Nan::NAN_METHOD_ARGS_TYPE)’:
../src/GNContext.cpp:877:81: warning: ‘v8::Local<v8::Value> Nan::MakeCallback(v8::Local<v8::Object>, v8::Local<v8::Function>, int, v8::Local<v8::Value>*)’ is deprecated [-Wdeprecated-declarations]
  877 |         Nan::MakeCallback(Nan::GetCurrentContext()->Global(), localCb, 1, cbArgs);
      |                                                                                 ^
In file included from ../src/GNContext.h:32,
                 from ../src/GNContext.cpp:28:
../node_modules/nan/nan.h:1026:46: note: declared here
 1026 |   NAN_DEPRECATED inline v8::Local<v8::Value> MakeCallback(
      |                                              ^~~~~~~~~~~~
../src/GNContext.cpp:886:81: warning: ‘v8::Local<v8::Value> Nan::MakeCallback(v8::Local<v8::Object>, v8::Local<v8::Function>, int, v8::Local<v8::Value>*)’ is deprecated [-Wdeprecated-declarations]
  886 |         Nan::MakeCallback(Nan::GetCurrentContext()->Global(), localCb, 1, cbArgs);
      |                                                                                 ^
In file included from ../src/GNContext.h:32,
                 from ../src/GNContext.cpp:28:
../node_modules/nan/nan.h:1026:46: note: declared here
 1026 |   NAN_DEPRECATED inline v8::Local<v8::Value> MakeCallback(
      |                                              ^~~~~~~~~~~~
../src/GNContext.cpp:916:81: warning: ‘v8::Local<v8::Value> Nan::MakeCallback(v8::Local<v8::Object>, v8::Local<v8::Function>, int, v8::Local<v8::Value>*)’ is deprecated [-Wdeprecated-declarations]
  916 |         Nan::MakeCallback(Nan::GetCurrentContext()->Global(), localCb, 1, cbArgs);
      |                                                                                 ^
In file included from ../src/GNContext.h:32,
                 from ../src/GNContext.cpp:28:
../node_modules/nan/nan.h:1026:46: note: declared here
 1026 |   NAN_DEPRECATED inline v8::Local<v8::Value> MakeCallback(
      |                                              ^~~~~~~~~~~~
../src/GNContext.cpp: In static member function ‘static Nan::NAN_METHOD_RETURN_TYPE GNContext::HelperLookup(Nan::NAN_METHOD_ARGS_TYPE)’:
../src/GNContext.cpp:992:81: warning: ‘v8::Local<v8::Value> Nan::MakeCallback(v8::Local<v8::Object>, v8::Local<v8::Function>, int, v8::Local<v8::Value>*)’ is deprecated [-Wdeprecated-declarations]
  992 |         Nan::MakeCallback(Nan::GetCurrentContext()->Global(), localCb, 1, cbArgs);
      |                                                                                 ^
In file included from ../src/GNContext.h:32,
                 from ../src/GNContext.cpp:28:
../node_modules/nan/nan.h:1026:46: note: declared here
 1026 |   NAN_DEPRECATED inline v8::Local<v8::Value> MakeCallback(
      |                                              ^~~~~~~~~~~~
In file included from ../src/GNContext.h:31,
                 from ../src/GNContext.cpp:28:
../src/GNContext.cpp: At global scope: