Add deferred option for cxy_ctl

[jaynel]

gen_stream used buffer striping that was driven by access. New concurrent process didn't execute until there was enough demand. This can be done programmatically by the user with maybe_execute_task.

Using a 'deferred' option so that up to a Max of spawns occurs, after which the spawning mechanism pauses until below Max before allowing the next to start. This involves maintaining Max processes running and a queue of deferred processes (the queue is the synchronization barrier).

Another option is to spawn as often as is needed, but to not start computation when over the Max. Processes could only proceed when given a signal to do so. This in itself is a form of synchronization.

The 2nd option allows for a more general implementation of synchronization barriers at the expense of more processes and more memory, but they are quicker to start running having already amortized the spawn cost.