Fast updates for signer

The signer has been designed to keep large zones signed where per run of the signer there is a significant amount of signatures per zone to be refreshed.
There are a number of performance and features;

• IXFR input for OpenDNSSEC allows only for one simultanous transfer to occur at the time.
• The entire zone file is written for each time a zone is getting signed.
• The signer performs multiple iterations over the full zone contents, even though only a few
  RRsets have been changed.