This commit adds better security for scroll requests in that they are now tied to a single user as
we only authorize the request that creates the scroll. This is accomplished by adding a
SearchOperationListener that listens for new scroll contexts and stores the authentication on the
ScrollContext. Then upon
retrieval of the search context for a query or fetch, the current authentication is compared to the
authentication that was present when the scroll context was created. If the current authentication
belongs to a different user, then a SearchContextMissingException will be thrown to prevent leaking
a valid vs invalid scroll id.
Additionally, signing of a scroll id is only performed when there is a older node in the cluster
that would expect the scroll id to be signed. Once this is backported to 5.x, we can remove this
bwc layer for 6.0/master.
Original commit: elastic/x-pack-elasticsearch@0e5dcafd32
2d893df7e9