public void modifyReindexing(ApplicationId id, UnaryOperator<ApplicationReindexing> modifications) {
        getTenant(id).getApplicationRepo().database().modifyReindexing(id, ApplicationReindexing.empty(), modifications);
    }
Gets the active Session for the given application id.

@return the active session, or null if there is no active session for the given application id.
configserver/src/main/java/com/yahoo/vespa/config/server/ApplicationRepository.java