MINOR: lists: Implement locked variations.

MINOR: lists: Implement locked variations.

Implement LIST_ADD_LOCKED(), LIST_ADDQ_LOCKED(), LIST_DEL_LOCKED() and
LIST_POP_LOCKED().

LIST_ADD_LOCKED, LIST_ADDQ_LOCKED and LIST_DEL_LOCKED work the same as
LIST_ADD, LIST_ADDQ and LIST_DEL, except before any manipulation it locks
the relevant elements of the list, so it's safe to manipulate the list
with multiple threads.
LIST_POP_LOCKED() removes the first element from the list, and returns its
data.