Make cleanup return a future[Unit] for the async caches
use toSeq instead of trimTo(0)
Add in two new options for the MultiTriggerCache:

High memory water mark when to flush

and when to crush down a value
Merge branch 'refs/heads/feature/branchF' into feature/ackOnEntry

Conflicts:
	summingbird-storm/src/main/scala/com/twitter/summingbird/storm/Constants.scala
	summingbird-storm/src/main/scala/com/twitter/summingbird/storm/StormPlatform.scala