* make CSRF protection the default on all non-SAFE methods
note: there currently is no way to call a CSRF-protected endpoint
without a form-encoded entity-body as that's the only place we get the
CSRF token from.
* simple CSRF token generation: just use the HMAC'd session id, no
generating a new random token per session then HMAC it
* use constant-time equal function to avoid timing attacks
* assert that a database secret is configured before hashing/validating
the CSRF token
* opt-out database manager from CSRF: The super-admin password serves
the purpose of a CSRF token in the database manager screens.
There is no request database to obtain the
secret and generate a CSRF token.
Christophe Matthieu