Fix retries causing constraint violation on MySQL with DAG Serialization

[kaxil]

The issue was caused because the rendered_task_instance_fields table did not have precision and hence causing _mysql_exceptions.IntegrityError.

closes #9148

Issue:

(Pdb) p session.query(RTIF).all()[-1].execution_date
datetime.datetime(2020, 6, 16, 20, 1, 30, tzinfo=Timezone('UTC'))
(Pdb)  p rttif.execution_date
datetime.datetime(2020, 6, 16, 20, 1, 29, 759742, tzinfo=Timezone('UTC'))
(Pdb)

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