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](https://github.com/envoyproxy/envoy/blob/02a526257a013e80bc508399446aac0234c3ec4e/source/common/common/hash.h#L35) 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