Make it possible to start the devtools after startup

[paulrouget]

No description provided.

[highfive]

cc @fitzgen

[jdm]

This has interesting implications, because every script thread has a direct channel to the devtools server, and is responsible for sending notifications about new globals. One possibility is that we could have the channels always exist and slowly fill up over time with nothing reading them; this would allow us to start the devtools server later and read the backlog of messages to end up in a reasonable state.

The other option would be to have script threads store an optional channel, and when it's time to start a server then the constellation distributes a channel to all script threads, who then send notifications about the globals that currently exist. We might need to add some kind of buffering somewhere for console.log output/JS exceptions/etc. that could be sent when connecting to a global, as well, so that developers are able to see output that occurred before the devtools server started?

[jdm]

We might be able to make it easier by having the script thread always talk to the constellation instead of directly to the devtools, which can then make decisions about buffering or discarding in a single central location.