[9.18] Fix missing RRSIG for CNAME with different slabheader order
The cachedb was missing piece of code (already found in zonedb) that would make lookups in the slabheaders to miss the RRSIGs for CNAME if the order of CNAME and RRSIG(CNAME) was reversed in the node->data.
(cherry picked from commit 5070c7f5)
Backport of !8675 (merged)
Edited by Ondřej Surý