Configure `Ssl` to be silent during certificate chain validation

This depends on https://github.com/Incubaid/ocaml-ssl/commit/7ae54282f2db8ce88b0bb25284cc0c975900ae73 which should first get into https://github.com/savonet/ocaml-ssl and a subsequent release of the OCaml `ssl` library, so this is not ready for merging yet.

* [ ] Get https://github.com/Incubaid/ocaml-ssl/commit/7ae54282f2db8ce88b0bb25284cc0c975900ae73 reviewed
* [ ] Send pull-request to https://github.com/savonet/ocaml-ssl
* [ ] Wait for new `ssl` release
* [ ] Wait for new `ssl` version available in https://github.com/ocaml/opam-repository