inspector: address race conditions

Stress tests uncovered 2 race conditions, when IO events happened during
V8 entering event loop on pause or during Node.js shutdown.

Fixes: nodejs/node#8669
PR-URL: https://github.com/nodejs/node/pull/8672


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>