[Reviewer: Matt] Sprout scaling down code

Matt please can you review my changes for sprout scaling down. This includes:
-  Code in main.cpp to trigger quiescing/unquiescing triggered by SIGQUIT/SIGUSR1
-  A new class (QuiescingManager) to coordinate quiescing.
-  A new class (ConnectionTrracker) to track TCP connections and shut them down. 
-  Some plumbing code in the stack module.

I've fully tested the two new classes. The code in main.cpp and the stack module will be live tested as part of the upcoming chef changes.