Changes file for feature4900

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+  o Minor features:
+
+    - Avoid hash-flooding denial-of-service attacks by using the secure
+      SipHash-2-4 hash function for our hashtables.  Without this
+      feature, an attacker could degrade performance of a targeted
+      client or server by flooding their data structures with a large
+      number of data entries all calculated to be stored at the same
+      hash table position, thereby degrading hash table
+      performance. With this feature, hash table positions are derived
+      from a randomized cryptographic key using SipHash-2-4, and an
+      attacker cannot predict which entries will collide.
+      Closes ticket 4900.