djb2CaseInsensitiveHash may cause excessive probing with Abseil hash tables

Title: djb2CaseInsensitiveHash may cause excessive probing with Abseil hash tables

Description: My colleagues who implemented the Abseil containers (@fowles and @sbenzaquen) mentioned that djb2CaseInsensitiveHash will likely cause excessive probing with absl::{flat,node}_hash_{set,map}. StringUtil uses this hash [1] in an absl::flat_hash_set [2]. Ideally this would be built on top of the absl::Hash framework [3] instead.

Relevant Links: [1] https://github.com/envoyproxy/envoy/blob/02a526257a013e80bc508399446aac0234c3ec4e/source/common/common/utility.cc#L472 [2] https://github.com/envoyproxy/envoy/blob/02a526257a013e80bc508399446aac0234c3ec4e/source/common/common/utility.h#L170 [3] https://abseil.io/docs/cpp/guides/hash