**Apache Airflow version**: 2.0.0


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

**Environment**: 

- **Cloud provider or hardware configuration**: local/aws
- **OS** (e.g. from /etc/os-release): Ubuntu 18.04.5 LTS
- **Kernel** (e.g. `uname -a`): 5.4.0-1032-aws
- **Install tools**: pip
- **Others**:

**What happened**:
I did a fresh Airflow 2.0.0 install. With this version, when I manually trigger a DAG, Airflow skips the next scheduled run.
<!-- (please include exact error messages if you can) -->

**What you expected to happen**:
Manual runs do not interfere with the scheduled runs prior to Airflow 2. 
<!-- What do you think went wrong? -->

**How to reproduce it**:
Create a simple hourly DAG. After enabling it and the initial run, run it manually. It shall skip the next hour. Below is an example, where the manual run with execution time of 08:17 causes the scheduled run with execution time of 08:00 to skip. 
![image](https://user-images.githubusercontent.com/26160471/103457719-c7193480-4d12-11eb-82cb-42efaedc9ef4.png)


airflow/models/dag.py
"""
        Returns a list of dag run execution dates currently running
        :return: List of execution dates
        """
        runs = DagRun.find(dag_id=self.dag_id, state=State.RUNNING)
        active_dates = []
        for run in runs:
            active_dates.append(run.execution_date)
        return active_dates
    @provide_session
    def get_num_active_runs(self, external_trigger=None, session=None):
        """
        Returns the number of active "running" dag runs
        :param external_trigger: True for externally triggered active dag runs
        :type external_trigger: bool
        :param session:
        :return: number greater than 0 for active dag runs
        """
        
        query = (
            session.query(func.count())
            .filter(DagRun.dag_id == self.dag_id)
            .filter(DagRun.state == State.RUNNING)
        )
        if external_trigger is not None:
            query = query.filter(DagRun.external_trigger == external_trigger)
        return query.scalar()
    @provide_session
    def get_dagrun(self, execution_date, session=None):