[profiler] Do the final sample dump on shutdown in the helper thread.

[profiler] Do the final sample dump on shutdown in the helper thread.

Reading the various fields on the profiler structure from the shutdown
thread is very racy; for example, we have no guarantee that we will see
a consistent GPtrArray structure for sorted_sample_events. It's better
to just let the helper thread empty the stat buffers when it shuts down.