CLEANUP: stream: explain why we queue the stream at the head of the server list

CLEANUP: stream: explain why we queue the stream at the head of the server list

In stream_add_srv_conn() MT_LIST_ADD() is used instead of MT_LIST_ADDQ(),
resulting in the stream being queued at the end of the server list. This
has no particular effect since we cannot dump the streams on a server,
and this is only used by "shutdown sessions" on a server. But it also
turns out to be significantly faster due to the shorter recovery from
the conflict with an adjacent MT_LIST_DEL(), thus it remains desirable
to use it, but at least it deserves a comment.

In addition to this, it's worth mentioning that this list should creates
extreme contention with threads while almost never used. It should be
made per-thread just like the global streams list.