[resolver] Avoid race conditions
    
    When sqla is building query results and sees an object that it already
    has in its session, it returns that object ignoring the actual row from
    query result. With that it tries to emulate repeatable read level of
    isolation. But in koschei, we sometimes need read committed isolation
    level in order to achieve mutual exclusion of side-effects (fedmsg,
    build scheduling). Expiring the objects in resolver forces their refresh,
    avoiding race conditions with scheduler (like described in #100 and #158).