Fixing HPACK header table resize issue

Summary: On resizing the header table down and then up again, a resize can be called against the underlying vector that actually sizes it down.  This causes a lot of things to break as the code that does the resizing assumes the underlying vector is only ever resized up.

Reviewed By: afrind

Differential Revision: D4613681

fbshipit-source-id: 35b61cab53d5bc097424d6c779f90b7fdea42002