inspector: Unify event queues

Current implementation tracks connected/disconnected status separately
which potentially introduces race condition.
This change introduces notion of session IDs and also posts
connect/disconnect events into the same queue as the messages. This way
Node knows what session given response belongs to and can discard
messages if the frontend for that session had disconnected.

This also fixes an issue when frontend was unable to attach to V8
instance that was running infinite loop.

PR-URL: https://github.com/nodejs/node/pull/7271


Reviewed-By: bnoordhuis - Ben Noordhuis <info@bnoordhuis.nl>