The Heap data structure is useful for our backoff system in addition to
scheduling queue. Move it to somewhere it can be consumed by both
systems and properly export needed names. Also adding unit tests
from client-go/tools/cache/heap.go.
- don't update nominatedMap cache when Pop() an element from activeQ
- instead, delete the nominated info from cache when it's "assumed"
- unit test behavior adjusted
- expose SchedulingQueue in factory.Config
