src: WIP use external array data to store pointer

Instead of assigning the class as an internal field, assign the memory
pointer as external array data.

NOTES:
- changes to node_internals.h and tls_wrap.cc are causing
  test-tls-ticket-cluster to die.

Still getting a few of the kinks worked out, but overall looking good. Also working on the performance tuning.

/cc @bnoordhuis, no idea what you'll have to say about this. :)