[AIRFLOW-4298] Stop Scheduler repeatedly warning "connection invalidated" (#5470)
[AIRFLOW-4298] Stop Scheduler repeatedly warning "connection invalidated" (#5470)

This appeared a lot more often with recent versions of SQLAlchemy and is
not a sign of any problem in the _initial_ case, so don't log the
message as a warning the first time we retry.
apache/airflow,cfei18/incubator-airflow,bolkedebruin/airflow,Acehaidrey/incubator-airflow,DinoCow/airflow,mtagle/airflow,cfei18/incubator-airflow,danielvdende/incubator-airflow,nathanielvarona/airflow,airbnb/airflow,airbnb/airflow,spektom/incubator-airflow,nathanielvarona/airflow,wooga/airflow,wooga/airflow,lyft/incubator-airflow,spektom/incubator-airflow,lyft/incubator-airflow,mrkm4ntr/incubator-airflow,dhuang/incubator-airflow,spektom/incubator-airflow,mtagle/airflow,Acehaidrey/incubator-airflow,nathanielvarona/airflow,nathanielvarona/airflow,mistercrunch/airflow,sekikn/incubator-airflow,bolkedebruin/airflow,dhuang/incubator-airflow,apache/incubator-airflow,lyft/incubator-airflow,Acehaidrey/incubator-airflow,dhuang/incubator-airflow,Acehaidrey/incubator-airflow,mistercrunch/airflow,wooga/airflow,sekikn/incubator-airflow,cfei18/incubator-airflow,sekikn/incubator-airflow,apache/airflow,cfei18/incubator-airflow,mrkm4ntr/incubator-airflow,cfei18/incubator-airflow,apache/airflow,Acehaidrey/incubator-airflow,airbnb/airflow,Fokko/incubator-airflow,Fokko/incubator-airflow,danielvdende/incubator-airflow,danielvdende/incubator-airflow,nathanielvarona/airflow,apache/airflow,wooga/airflow,airbnb/airflow,DinoCow/airflow,apache/incubator-airflow,bolkedebruin/airflow,mrkm4ntr/incubator-airflow,Fokko/incubator-airflow,DinoCow/airflow,Acehaidrey/incubator-airflow,mtagle/airflow,apache/airflow,apache/incubator-airflow,mistercrunch/airflow,apache/airflow,bolkedebruin/airflow,bolkedebruin/airflow,wileeam/airflow,wileeam/airflow,danielvdende/incubator-airflow,mtagle/airflow,apache/incubator-airflow,nathanielvarona/airflow,sekikn/incubator-airflow,mrkm4ntr/incubator-airflow,spektom/incubator-airflow,lyft/incubator-airflow,danielvdende/incubator-airflow,wileeam/airflow,DinoCow/airflow,danielvdende/incubator-airflow,wileeam/airflow,mistercrunch/airflow,Fokko/incubator-airflow,cfei18/incubator-airflow,dhuang/incubator-airflow