Summary: By modifying `IntervalSet` a bit we can make it so it takes a `folly::small_vector` as the container. We expect that for real traffic there will not generally be a lot of ACK blocks per frame, so optimize for that.
Reviewed By: siyengar
Differential Revision: D18919975
fbshipit-source-id: 199a2ea9ba5003382e2d7d99fc7a6de7e8aafdca