Fix handling of "quiet" threads (or threads at exit)

[rburchell]

Right now, chunks are only submitted when the chunk is full, or systrace_deinit is called. This is bad for a few reasons.

Firstly: it does not handle abrupt termination
Secondly: it does not handle quiet threads (that only report 1-2 events)
Thirdly: it does not handle threads that report a lot, but quit at some point

I think we need to change the way chunk allocation works to fix this, but changing that may fix a few other problems at the same time.

Suggested workflow:

• Client asks traced for a chunk name
• traced hands it one, records it as in use
• Client continues to notify traced of full chunks (but we now also use this to get a new chunk allocated at the same time

Every N (and at startup):

• traced looks over open chunks
  • If chunk PID is dead, or chunk TID is dead, process (and free) chunk

Note that if a chunk is kept open by a thread, but the thread is not reporting much, this is still a problem (until termination). I don't see this as being a problem, but if it turns out to be one, we can always look at a solution for that. I'm a little wary about trying to handle that case now, because it could likely get ugly fast.