Fastly VCL
あとで使用するための変数を格納する場所としても使われる
Request headers are also frequently used as variables to store strings for later use. Fastly VCL also offers local variables as an alternative for storing temporary values.
下記のように書かれているため、新たに set bereq.http.{NAME} とは書けなさそう？
Before Fastly invokes vcl_miss or vcl_pass, we copy the contents of req.http.* into a new set of headers called bereq.http. This header list can then be modified without affecting the original request received from the client.