Move ComponentBuildTrace creation from before_commit to before_flush
    
    Since ComponentBuildTrace(s) get created with db_session.commit() call,
    is is not possible to commit more items in bulk if they already have been flushed.
    
    Current unit-tests' setup can be significantly sped up if items can be quickly
    flushed on the fly and bulk-commited only once at the end. Moreover in general it
    seems more appropriate/safer to handle this in before_flush as any implicit
    or accidental flush could cause new build traces not to be created at all. As flush
    is implicitly called before every commit anyway, this change shouldn't pose any harm.