Handle connection failure and retry connecting instead of having to maitain a pingback client/server.

I'm aiming to upstream a bunch of simulator code to devtools/firefox.
The pingback is one simulator part that, I think, won't be easy to upstream.
I'm not sure it worth the cost, whereas I can easily imagine submitting a patch to offer a retry-until-timeout feature to DebuggerClient.

I'm not sure this patch will fix any reported issue, but it will definitely simplify b2g desktop startup workflow and eventually avoid some bugs with extremelly strict firewalls.