Fix performance by skipping XML debug when debugging is disabled

Summary:
The XML bodies of request and response can be big. We're using
temp files to write them into the debug logs, and we did that
even when debugging is disabled.

Now we skip that slow code if debugging is disabled. I also unified
logging code for XML response and request.

Reviewers: #syncroton_developers, mollekopf

Reviewed By: #syncroton_developers, mollekopf

Subscribers: #syncroton_developers

Differential Revision: https://git.kolab.org/D4430