Follow-up from "Add session_prefix rules and implement matching cookie for classify"

The following discussion from gitlab-org/cells/http-router!233 (merged) should be addressed:

• @tkuah started a discussion:

  non-blocking In order to test this correctly, we had to have a session prefix of cell-1- (note the trailing -)

  We do need the - delimiter because it turns out the session id is hex, so the regex can get confused (e.g. cookie value of cell-1003asdasdasds will return cell-1003 by mistake when we want cell-1.

  Adding the delimiter in the Rails/GDK config feels off though, we should add the delimiter in code instead (lib/gitlab/sessions/redis_store.rb)