Ability to resync access to a room after losing your local replica

Currently if you lose your local replica of a room the best bet is to have your users rejoin that room. It would be good to have a better way to resync the existent room

The issue does not name any files, tests, or protocol entry points. First clarify how a user should regain access to an existing room after losing its local replica and what state must be synchronized. Done should be a specified, implementable resynchronization mechanism that avoids requiring users to rejoin.

Written by the indexing model from the issue text.