make Sozu-Id header field name configurable

[Keksoj]

for now, the request id is return a header that looks like this:

Sozu-Id: 01JA8XRT52ZSZSCGY54F10C807

We may want to make the header field name (in this case, Sozu-Id) configurable.

[Keksoj]

This will be relevant if we want to make Sōzu tracing-compatible. We may then want to concatenate the header field values in case of several Sōzus in front of each other, and have something like this:

Sozu-Id: 01JA8XRT52ZSZSCGY54F10C807,01J98TZTJWRZKK3QR6T726KNTK,01J0BGBSKSJ9C78NTBGKD2G58A