This commit removes the warn-date from warning headers. Previously we
were stamping every warning header with when the request
occurred. However, this has a severe performance penalty when
deprecation logging is called frequently, as obtaining the current time
and formatting it properly is expensive. A previous change moved to
using the startup time as the time to stamp on every warning header, but
this was only to prove that the timestamping was expensive. Since the
warn-date is optional, we elect to remove it from the warning
header. Prior to this commit, we worked in Kibana to make the warn-date
treated as optional there so that we can follow-up in Elasticsearch and
remove the warn-date. This commit does that.
715719ee3b