This change migrates all of the xpack code to use the new ThreadContext when
dealing with headers and context data. For the most part this is a simple
cutover, but there are some things that required special casing. The internal
actions that executed by a user's requests need to forcefully drop the context
and set the system user. The workaround for this will be improved in a followup.
Additionally, the RequestContext still lives on due to the OptOutQueryCache,
which requires some core changes to fix this issue.
Original commit: elastic/x-pack-elasticsearch@87d2966d93
