BUG/MEDIUM: lists: Lock the element while we check if it is in a list.

BUG/MEDIUM: lists: Lock the element while we check if it is in a list.

In MT_LIST_TRY_ADDQ() and MT_LIST_TRY_ADD() we can't just check if the
element is already in a list, because there's a small race condition, it
could be added  between the time we checked, and the time we actually set
its next and prev, so we have to lock it first.

This is required to address issue #958.

This should be backported to 2.3, 2.2 and 2.1.