Fix worker_threads support

[Qard]

This makes node-rdkafka work correctly with worker_threads. There were several issues which needed resolving and some changes which were needed to prevent data being shared across threads and to ensure everything is cleaned up completely when the thread exits.

Changes included:

• Use NODE_MODULE_CONTEXT_AWARE to init module
• No globals, shared data needs to live in PerIsolateData.
• No using uv_default_loop(), need to get the loop owned by the current thread.
• Dispatcher::Deactivate() on destruct to stop uv_async_t so thread can teardown loop safely
• Connection and Topic types need cleanup hooks to ensure they are deleted when the thread is torn down.
• Construct Conf with correctly-sized type so destructor and sub-class fields aren't overwriting random memory
• Give AsyncWorker types unique AsyncResource names for better debugging

[mcollina]

@GaryWilber can you take a look?

[DrJume]

Currently, no native-backed Node.js Kafka library seems to support worker threads...