This fixes the OriginalIP and RequestUserID in the main featurebase
package, and the Access and Refresh tokens, the UserInfo, and the
[]string of Indexes passed with context.Context(s) in the authn package.
An empty struct was used for all of these keys (and relevant helper
functions we added) to avoid allocations where possible while still
using the context functionality.
Some of the logic in the server.GetIndexes function was fixed.
(cherry picked from commit 0f5a56c958)
