Wait till nextTick to pop() the next object

Wait till nextTick to pop() the next item.

Previously we were popping the item, then waiting for nextTick before
calling the callback.  Now, we call the pop callback immediately with a
new item but the "finished()" function to signify completion waits till
nextTick to give the pop() callback its next item.

The way that cimpler is using this module made it beneficial to have
`push() -> pop(callback)` incur no nextTick delay, and leave that for
the `finished() -> pop callback` transition.