A few debugplugin updates

Merging master into this branch made this diff a bit unusable.
The relevant (tiny) updates to the debugplugin branch itself are:

* https://github.com/DominikTo/sabre-dav/commit/3455968540ea5d4e74468a52d2ada56060bf8558
* https://github.com/DominikTo/sabre-dav/commit/bd03887e5df22d20da28465e3966d5f10814e5f6
* https://github.com/DominikTo/sabre-dav/commit/c7cc46462aad1a23987a6f61f09ac98f6b3d25df
* https://github.com/DominikTo/sabre-dav/commit/715fa0cf7da2971dd43bcc78233fed1e2722a12e
* https://github.com/DominikTo/sabre-dav/commit/0dd9b8a6e5b7db3334e30a8a4bc1ad34013eac85
should be private