Log Message:
nghttp2: updated to 1.28.0

nghttp2 v1.28.0
lib: Add nghttp2_error_callback2
build: Add deprecation warning when spdylay support is enabled
Switch to clang-format-5.0
examples: Make client and server work with libevent-2.1.8
third-party: Update neverbleed
integration: Fix issues reported by the go vet tool.
nghttpx: Fix affinity retry
nghttpx: Fix stalled backend connection on retry
nghttpx: Cookie based session affinity
nghttpx: Expose additional TLS related variables to mruby and accesslog