#45 Erease private key data from CMP_CTX as soon as no longer needed

[David von Oheimb]

This is a security enhancement proposed by Philipp Löffler.

In particiular, private keys just needed for POPO generation can be erased immediately thereafter.
All private keys of client might even be erased temporarily before waiting for next poll request and then re-read from their respective source (e.g., password-protected file).