msg/simple: wait dispatch_queue until all pipes closed

msg/simple: wait dispatch_queue until all pipes closed

Now we use dispatch_queue.wait to wait for SimpleMessenger shutdown,
but we need to ensure DispatchQueue can process event after Accepter down,
Otherwise accepter may continue to accept new connection which may queue
new item. so we can't rely on DispatchQueue now.

Introduce stop_cond and stop flag to indicate this function like
AsyncMessenger did

Fixes: http://tracker.ceph.com/issues/16472
Signed-off-by: Haomai Wang <haomai@xsky.com>