Parallel signing

Three separate commits:

• first one might be important also as a bugfix(?), avoiding overwriting shared buffers while signing rrsets in parallel threads, which is already the case in onlinesign
• second one implements parallel signing of the zone. The speedup observed was almost proportional.
• third one implements parallel signing of changes to the zone, but its benefit is very questionable because the updates are veeeery slow outside of the signing routine.

Fixes #186 (closed)