contextify: tie lifetimes of context & sandbox

When the previous set of changes (bfff07b4) it was possible to have the
context get garbage collected while sandbox was still live. We need to
tie the lifetime of the context to the lifetime of the sandbox.

This is a backport of #5786 to v5.x.

Ref: https://github.com/nodejs/node/pull/5786
PR-URL: https://github.com/nodejs/node/pull/5800


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>