code: static RPC<TaskGetKey> start( H2ONode target, Key key ) {
      // Do we have an old TaskGetKey in-progress?
      RPC<TaskGetKey> old = TGKS.get(key);
      if( old != null ) return old;
      // Make a new TGK.
      RPC<TaskGetKey> rpc = new RPC(target,new TaskGetKey(key),1.0f);
      if( (old=TGKS.putIfMatchUnlocked(key,rpc,null)) != null )
        return old;               // Failed because an old exists
      rpc.setTaskNum().call();    // Start the op
      return rpc;                 // Successful install of a fresh RPC
    }
docs: Start an RPC to fetch a Value, handling short-cutting dup-fetches