DOC: internals: update the SSL architecture schema

This commit adds the new fields added to the ckch_inst structure in
order to manage the backend certificate hot update (GitHub #427) and the
bug of the default certificate update (GitHub #1143).
diff --git a/doc/internals/ssl_cert.dia b/doc/internals/ssl_cert.dia
index 5084830..422d306 100644
--- a/doc/internals/ssl_cert.dia
+++ b/doc/internals/ssl_cert.dia
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