cam: Move temporary queue from stack to softc

[imp]

Summary

For the async thread, as well as the per-cpu done threads, move the
queue we copy all the requests into to the softc. This allows us to
dispaly this otherwise hidden state form the debugger.

Sponsored by: Netflix

[mav]

My only complaint is that it puts the queue into the same cache line as the main queue, that may be modified by writers. But if you really need it for debugging, it could be understood.

PS: You seem to be doing STAILQ_INIT() twice for doneqs.