[orc-rt] Fix some Session::shutdown bugs. (#177528)
All calls to Session::shutdown were enquing their on-shutdown-complete
callbacks in Session's ShutdownInfo struct, but this queue is only
drained once by the thread that initiates shutdown. After the queue is
drained, subsequent calls to Session::shutdown were enquing their
callbacks in a queue that would never be drained.
This patch updates Session::shutdown to check whether shutdown has
completed already and, if so, run the on-shutdown-complete immediately.
This patch also fixes a concurrency bug: Session::shutdownComplete was
accessing SI->OnCompletes outside the session mutex, but this could lead
to corruption of SI->OnCompletes if a concurrent call to
Session::shutdown tried to enqueue a new callback to SI->OnCompletes
concurrently. This has been fixed by moving the SI->OnCompletes queue to
a new variable under the Session mutex, then draining the new queue
outside the mutex. (No testcase yet: this was discovered by observation,
and replicating the bug would depend on timing).
