backend: build reattaching: avoid doubled fc.update() call
    
    For each reattached build we called mark_running twice, this both caused
    doubled 'starting build: %s' log info message and calls to frontend via
    fc.update() (the second one was just wasting of traffic).
    
    Move the 'self.reattach' condition to _announce_start so it is easier to
    understand and always call _announce_start() ASAP in Worker.run() (no
    matter we reattach or not).
    
    While we are on it, rename mark_running to internal _mark_running.
    
    Merges: #1116