Early abort for forEach, peek multiple

Adds the capability to peek at multiple entries at the head of the
queue. I didn't have to change QueueFile, as its forEach already can
avoid reading bytes, but this optimization to abort early saves some
object creation.

Not a breaking API change.
This isn't a "last-test" cleanup, it's that QueueFile cannot operate on empty files.  I'll change it a bit to make this more clear.