lib,src: clean up ArrayBufferAllocator

Remove the direct dependency on node::Environment (which is per-context)
from node::ArrayBufferAllocator (which is per-isolate.)

Contexts that want to toggle the zero fill flag, now do so through a
field that is owned by ArrayBufferAllocator.  Better, still not great.

PR-URL: https://github.com/nodejs/node/pull/7082


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>