**Apache Airflow version**: 2.0.0


**Kubernetes version (if you are using kubernetes)** (use `kubectl version`):

**Environment**:

- **Cloud provider or hardware configuration**:
- **OS** (e.g. from /etc/os-release):
- **Kernel** (e.g. `uname -a`):
- **Install tools**:
- **Others**:

**What happened**:

I have the following DAG:

```py
from datetime import datetime, timedelta

from airflow import DAG
from airflow.operators.bash_operator import BashOperator

with DAG(
    dag_id="sla_trigger",
    schedule_interval="* * * * *",
    start_date=datetime(2020, 2, 3),
) as dag:
    BashOperator(
        task_id="bash_task",
        bash_command="sleep 30",
        sla=timedelta(seconds=2),
    )

```

And in my understanding this dag should result in SLA miss every time it is triggered (every minute). However, after few minutes of running I don't see any SLA misses...

**What you expected to happen**:

I expect to see SLA if task takes longer than expected.

**How to reproduce it**:

Use the dag from above.

**Anything else we need to know**:

N/A

tests/jobs/test_scheduler_job.py
dag2.sync_to_db(session=session)
        ti1 = TaskInstance(task1, DEFAULT_DATE)
        ti1.state = State.QUEUED
        session.merge(ti1)
        session.commit()
        executor.event_buffer[ti1.key] = State.FAILED, None
        scheduler._process_executor_events(session=session)
        ti1.refresh_from_db()
        assert ti1.state == State.QUEUED
        mock_task_callback.assert_called_once_with(
            full_filepath='/test_path1/',
            simple_task_instance=mock.ANY,
            msg='Executor reports task instance '
            '<TaskInstance: test_process_executor_events.dummy_task 2016-01-01 00:00:00+00:00 [queued]> '
            'finished (failed) although the task says its queued. (Info: None) '
            'Was the task killed externally?',
        )
        scheduler.processor_agent.send_callback_to_execute.assert_called_once_with(task_callback)
        scheduler.processor_agent.reset_mock()
        
        ti1.state = State.SUCCESS
        session.merge(ti1)
        session.commit()
        executor.event_buffer[ti1.key] = State.SUCCESS, None
        scheduler._process_executor_events(session=session)
        ti1.refresh_from_db()
        assert ti1.state == State.SUCCESS
        scheduler.processor_agent.send_callback_to_execute.assert_not_called()
        mock_stats_incr.assert_called_once_with('scheduler.tasks.killed_externally')
    def test_process_executor_events_uses_inmemory_try_number(self):