Replace GetIsolate() by Isolate::GetCurrent()

[backes]

Replace any call to Object::GetIsolate() by Isolate::GetCurrent().
This does not change anything, as Object::GetIsolate() is already implemented as Isolate::GetCurrent() under the hood.

Medium term, some of the new calls could / should be replaced by passing an existing isolate pointer, which can be more efficient depending on usage patterns and the platform that we execute on.

One drive-by fix: Replace linked_requests_data.As<Array>() by linked_requests_data.As<Value>().As<Array>() to fix debug builds.

[backes]

@pthier PTAL

Not sure why compilation did not fail when calling these deprecated methods; I fixed everything that failed in #219. This now removes all remaining calls so we can actually remove the deprecated API.

[pthier]

Thanks, LGTM