[WIP] dont re-read on heartbeat

[saguziel]

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Description

Heartbeat sleep shouldn't need to read DB state, it's good enough to base it on the original time.

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[ashb]

I've got a similar change that I'm working on also imrpvoes the heartbeating of scheduler and job. Unless you feel strongly I'll close this - related to https://issues.apache.org/jira/browse/AIRFLOW-5615

Actually this change is slightly different, so both are probably worth-while.

[ashb]

We need to keep this though, see https://issues.apache.org/jira/browse/AIRFLOW-5102

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.