Upgrade the rapidjson library to the latest version.
This gives us better error reporting, and allows us to
remove two backports from upstream.

Since there has not been any official release in the last
four years, we have included the latest available commit
from https://github.com/Tencent/rapidjson/

Change-Id: I7a80693b108a1f3327a09bf138812c9f1aaa5bc9