Currently the SessionCatalog interface, specifically scanSessions is overly broad on what it allows callers to do in terms of state mutation. This function can be spit out into the following:
killSessions: which kills sessions with a given predicate

findExpiredParentSessions: Returns the set of parent session IDs whose last checkout time is earlier than some value

scanSessionsReadOnly: scans a read only list of sessions and provides the caller a const view of an ObservableSession.