[AIRFLOW-2777] speed up dag.sub_dag(...)
[AIRFLOW-2777] speed up dag.sub_dag(...)

previous version created the subdag by copying
over all the tasks, and
then filtering them down. it's a lot faster if we
only copy over the
tasks we need

Closes #3621 from abdul-stripe/faster-subdag

nathanielvarona/airflow,cfei18/incubator-airflow,sid88in/incubator-airflow,mrkm4ntr/incubator-airflow,apache/airflow,wooga/airflow,danielvdende/incubator-airflow,edgarRd/incubator-airflow,wileeam/airflow,spektom/incubator-airflow,artwr/airflow,lyft/incubator-airflow,andyxhadji/incubator-airflow,apache/incubator-airflow,fenglu-g/incubator-airflow,r39132/airflow,fenglu-g/incubator-airflow,mtagle/airflow,spektom/incubator-airflow,gtoonstra/airflow,mistercrunch/airflow,akosel/incubator-airflow,jgao54/airflow,airbnb/airflow,danielvdende/incubator-airflow,bolkedebruin/airflow,malmiron/incubator-airflow,criccomini/airflow,DinoCow/airflow,sid88in/incubator-airflow,danielvdende/incubator-airflow,adamhaney/airflow,adamhaney/airflow,DinoCow/airflow,apache/airflow,r39132/airflow,wooga/airflow,bolkedebruin/airflow,bolkedebruin/airflow,jgao54/airflow,sid88in/incubator-airflow,mrkm4ntr/incubator-airflow,wileeam/airflow,cfei18/incubator-airflow,criccomini/airflow,danielvdende/incubator-airflow,cfei18/incubator-airflow,edgarRd/incubator-airflow,mtagle/airflow,Acehaidrey/incubator-airflow,cfei18/incubator-airflow,nathanielvarona/airflow,cfei18/incubator-airflow,danielvdende/incubator-airflow,nathanielvarona/airflow,apache/incubator-airflow,mistercrunch/airflow,wooga/airflow,dhuang/incubator-airflow,mtagle/airflow,malmiron/incubator-airflow,bolkedebruin/airflow,mrkm4ntr/incubator-airflow,mistercrunch/airflow,apache/airflow,sid88in/incubator-airflow,nathanielvarona/airflow,bolkedebruin/airflow,mistercrunch/airflow,apache/airflow,lyft/incubator-airflow,gtoonstra/airflow,airbnb/airflow,criccomini/airflow,nathanielvarona/airflow,r39132/airflow,apache/airflow,airbnb/airflow,fenglu-g/incubator-airflow,sekikn/incubator-airflow,jgao54/airflow,artwr/airflow,akosel/incubator-airflow,akosel/incubator-airflow,Fokko/incubator-airflow,gtoonstra/airflow,Acehaidrey/incubator-airflow,Acehaidrey/incubator-airflow,gtoonstra/airflow,artwr/airflow,r39132/airflow,Fokko/incubator-airflow,edgarRd/incubator-airflow,dhuang/incubator-airflow,lyft/incubator-airflow,airbnb/airflow,andyxhadji/incubator-airflow,DinoCow/airflow,fenglu-g/incubator-airflow,malmiron/incubator-airflow,dhuang/incubator-airflow,artwr/airflow,sekikn/incubator-airflow,Fokko/incubator-airflow,spektom/incubator-airflow,andyxhadji/incubator-airflow,sekikn/incubator-airflow,dhuang/incubator-airflow,nathanielvarona/airflow,andyxhadji/incubator-airflow,jgao54/airflow,cfei18/incubator-airflow,malmiron/incubator-airflow,Acehaidrey/incubator-airflow,apache/incubator-airflow,akosel/incubator-airflow,wileeam/airflow,Acehaidrey/incubator-airflow,apache/airflow,adamhaney/airflow,edgarRd/incubator-airflow,danielvdende/incubator-airflow,DinoCow/airflow,wooga/airflow,apache/incubator-airflow,wileeam/airflow,mtagle/airflow,Acehaidrey/incubator-airflow,lyft/incubator-airflow,spektom/incubator-airflow,Fokko/incubator-airflow,mrkm4ntr/incubator-airflow,sekikn/incubator-airflow,criccomini/airflow,adamhaney/airflow