Update Data structure survey

Data-structure-survey.md

@@ -131,6 +131,8 @@ Ondrej: there's work to do on the isc_ht:
 2. there's a branch, where I implemented robin-hood hashing, but I never finished because of 1)
 3. the resizing was added only recently, we still miss shrinking (should be easy enough to do)
 
+Ondrej: dns_adb was recently refactored, it used a custom hashtable before
+
 # `isc/pool`
 
 A mechanism for sharing a small pool of fungible objects among a large