Improve synth-from-dnssec NSEC search algorithm
Take the signer for NSEC RRSIGs and use that name to find a tree in which to store and find the NSECs. Currently the NSECs are stored in a single tree which means the search can miss NSECs which a present in the cache. This sort of structure will also be needed for NSEC3 lookups when we extend synth-from-dnssec to use NSEC3's.