DevTools not properly shutting down between reloads in some cases

[bvaughn]

Noticed this while testing the recently added "highlight updates" feature. It seems like relying on the content script's "disconnect" to shutdown things is too late, and we seem to be leaving at least an Agent (possibly more) connected.

I think we may want to add some cleanup code to our initialization path (of the backend) to handle the case where a previous backend instance didn't get to finish shutting down.

[andrelmchaves]

@bvaughn Could you confirm this is still an issue?