Use a single room per server-user pair for events#3910

IanCal wants to merge 6 commits intomaincardstack/boxel:mainfrom

cs-9552-refactor-realm-event-sending-to-use-realm-server-sessioncardstack/boxel:cs-9552-refactor-realm-event-sending-to-use-realm-server-sessionCopy head branch name to clipboard