Adding next parameter to processors.

This allows processors to be used in intrusive data structures.
This would be useful for the processor queues, like the global
run-queue and the queues that are used inside the mutexes
and condition variables.
MPSC for locks, fixing MPSC node in processors.

The locks are now implemented with the MPSC queue, the nodes that
were intrusive in the processors won't work now because the tasks
already were intrusive. So now we just put a regular node in there,
and manually keep the contents in sync.

The benefits are still good: the queues that hold the processors
in theory shouldn't need any extra allocations.