Replace closed_slots with indirection

The closed slot system was a solution to manage the possiblity that a
pcb might have been closed by LwIP while operations were still pending
on the async thread. Replace this system with double indirection over
the pcb pointers, now owned solely by LwIP. This closes a possible hole
where a closed_slot may have been re-used while still held by an
AsyncClient when under heavy load.

Author: willmmiles