stream-ssl: Make changing keys and certificate at runtime reliable.
OpenSSL is picky about the order in which keys and certificates are
changed: you have to change the certificate first, then the key. It
doesn't document this, but deep in the source code, in a function that sets
a new certificate, it has this comment:
/* don't fail for a cert/key mismatch, just free
* current private key (when switching to a different
* cert & key, first this function should be used,
* then ssl_set_pkey */