Make sure EventPipe streaming thread won't write session->streaming_thread after...

Make sure EventPipe streaming thread won't write session->streaming_thread after session free. (#58710)

In case where ep_disable is called by a different thread (close IPC command)
there was a race between streaming threads setting
session->streaming_thread to NULL and IPC command triggering a call
to disable_holding_lock and freeing session.

Resetting the streaming_thread in streaming thread must happens before
it signals its shutdown event to prevent the race.

Co-authored-by: lateralusX <lateralusx.github@gmail.com>