Update to 2.1.9
    
    Issues solved:
    - OPENDNSSEC-955: Prevent concurrency between certain valid
                      PKCS#11 HSM operations to avoid some keys
                      to be (transiently) unavailable.
    - OPENDNSSEC-956: Harden signing procedure to still sign zones
                      for which there are unused keys specified in
                      the zone which are unavailable.
    
    Known issue:
    - OPENDNSSEC-957: Signer daemon stops with failure exit code
                      even when no error occured.