Multithreaded signing support for OpenDNSSEC

We have changed the design so that RRsets are added to a signing queue, where a pool of signer threads (called drudgers) grab a signing task and perform it. With the SCA6000 HSM we now reach maximum performance, meaning OpenDNSSEC can do a 13.000+ signatures per second. The .se zone can now be signed in 2 minute 50 seconds, of which 1 minute 14 seconds are signing operations.