<!--

Welcome to Apache Airflow!  For a smooth issue process, try to answer the following questions.
Don't worry if they're not all applicable; just try to include what you can :-)

If you need to include code snippets or logs, please put them in fenced code
blocks.  If they're super-long, please use the details tag like
<details><summary>super-long log</summary> lots of stuff </details>

Please delete these comment blocks before submitting the issue.

-->

<!--

IMPORTANT!!!

PLEASE CHECK "SIMILAR TO X EXISTING ISSUES" OPTION IF VISIBLE
NEXT TO "SUBMIT NEW ISSUE" BUTTON!!!

PLEASE CHECK IF THIS ISSUE HAS BEEN REPORTED PREVIOUSLY USING SEARCH!!!

Please complete the next sections or the issue will be closed.
These questions are the first thing we need to know to understand the context.

-->

**Apache Airflow version**: 2.0.2


**Environment**: 

- **Cloud provider or hardware configuration**:
- **OS** (e.g. from /etc/os-release): Ubuntu 18.04.2 LTS
- **Kernel** (e.g. `uname -a`): Linux datadumpprod2 4.15.0-54-generic #58-Ubuntu SMP Mon Jun 24 10:55:24 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux
- **Install tools**: Docker


**What happened**:

After task execution is done but process isn't finished yet, heartbeat callback kills the process because falsely detects external change of state.

```
[2021-06-02 20:40:55,273] {{taskinstance.py:1532}} INFO - Marking task as FAILED. dag_id=<dag_id>, task_id=<task_id>, execution_date=20210602T104000, start_date=20210602T184050, end_date=20210602T184055
[2021-06-02 20:40:55,768] {{local_task_job.py:188}} WARNING - State of this instance has been externally set to failed. Terminating instance.
[2021-06-02 20:40:55,770] {{process_utils.py:100}} INFO - Sending Signals.SIGTERM to GPID 2055
[2021-06-02 20:40:55,770] {{taskinstance.py:1265}} ERROR - Received SIGTERM. Terminating subprocesses.
[2021-06-02 20:40:56,104] {{process_utils.py:66}} INFO - Process psutil.Process(pid=2055, status='terminated', exitcode=1, started='20:40:49') (2055) terminated with exit code 1
```
This happens more often when mini scheduler is enabled because in such case the window for race condition is bigger (time of execution mini scheduler).

**What you expected to happen**:

Heartbeat should allow task to finish and shouldn't kill it.

**How to reproduce it**:

As it's a race condition it happens randomly but to make it more often, you should have mini scheduler enabled and big enough database that execution of mini scheduler takes as long as possible. You can also reduce heartbeat interval to minimum.


airflow/models/taskinstance.py
from airflow.configuration import conf
from airflow.exceptions import (
    AirflowException,
    AirflowFailException,
    AirflowRescheduleException,
    AirflowSensorTimeout,
    AirflowSkipException,
    AirflowSmartSensorException,
    AirflowTaskTimeout,
)
from airflow.models.base import COLLATION_ARGS, ID_LEN, Base
from airflow.models.log import Log
from airflow.models.taskfail import TaskFail
from airflow.models.taskreschedule import TaskReschedule
from airflow.models.variable import Variable
from airflow.models.xcom import XCOM_RETURN_KEY, XCom
from airflow.plugins_manager import integrate_macros_plugins
from airflow.sentry import Sentry
from airflow.stats import Stats
from airflow.ti_deps.dep_context import DepContext
from airflow.ti_deps.dependencies_deps import REQUEUEABLE_DEPS, RUNNING_DEPS
from airflow.utils import timezone
from airflow.utils.email import send_email
from airflow.utils.helpers import is_container
from airflow.utils.log.logging_mixin import LoggingMixin
from airflow.utils.net import get_hostname
from airflow.utils.operator_helpers import context_to_airflow_vars
from airflow.utils.platform import getuser
from airflow.utils.session import provide_session
from airflow.utils.sqlalchemy import UtcDateTime, with_row_locks