this improves the performance of looking for NSEC and RRSIG(NSEC)
records in the cache by skipping lots of nodes in the main trees
in the cache without these records present. This is a simplified
version of previous_closest_nsec() which uses the same underlying
mechanism to look for NSEC and RRSIG(NSEC) records in authorative
zones.
The auxilary NSEC tree was already being maintained as a side effect
of looking for the covering NSEC in large zones where there can be
lots of glue records that needed to be skipped. Nodes are added
to the tree whenever a NSEC record is added to the primary tree.
They are removed when the corresponding node is removed from the
primary tree.
Having nodes in the NSEC tree w/o NSEC records in the primary tree
should not impact on synth-from-dnssec efficiency as that node would
have held the NSEC we would have been needed to synthesise the
response. Removing the node when the NSEC RRset expires would only
cause rbtdb to return a NSEC which would be rejected at a higher
level.
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