js: Properly manage workers when useWorkers is called twice

Before this change, calling useWorkers twice would allocate new workers
but keep old workers around; this resulted in old workers leaking when
the new count was larger; when the new count was smaller, we would still
maintain the worker count but recreate some workers and leak older ones.

Instead we now only create more workers if more workers are requested,
and if the requested count is smaller than current worker count, we ask
some existing workers (nicely) to close by sending a dummy message.

Because messages are processed in order, and processing the worker
response does not access global workers[], it should be safe to call
useWorkers(0) at any point without disrupting already dispatched
requests. This also provides a clean way to reclaim all resources used
by the workers, including WebAssembly memory.