**Apache Airflow version**:
2.0.1

**Kubernetes version (if you are using kubernetes)** (use `kubectl version`):
Not applicable

**Environment**:

- **Cloud provider or hardware configuration**: None
- **OS** (e.g. from /etc/os-release): Ubuntu
- **Kernel** (e.g. `uname -a`): 
- **Install tools**:
- **Others**:

**What happened**:

1. Ran airflow db init on mysql in parallel in 2 command lines. Only one command did the migrations, the other one waited. But connections were inserted twice. I would like them not to be added twice.
2. Ran airflow db init on postgres in parallel in 2 command lines. Both command lines started doing migrations on the same db in parallel. I would like one command to run, the other to wait.

**What you expected to happen**:

1. For MySQL. Connections and other config objects to be inserted only once.
2. For Postgres. Only one migration can be performed in the same time for the same db.

**How to reproduce it**:
Scenario 1:
Setup Airflow so that it uses MySQL.
Run `airflow init db` in 2 command lines, side by side.

Scenario 2:
Setup Airflow so that it uses Postgres.
Run `airflow init db` in 2 command lines, side by side.

**Anything else we need to know**:
This problem occurs every time.
airflow/utils/db.py
"""
    Drops all airflow models.
    :param connection: SQLAlchemy Connection
    :return: None
    """
    from airflow.models.base import Base
    
    
    chart = Table('chart', Base.metadata)
    chart.drop(settings.engine, checkfirst=True)
    user = Table('user', Base.metadata)
    user.drop(settings.engine, checkfirst=True)
    users = Table('users', Base.metadata)
    users.drop(settings.engine, checkfirst=True)
    dag_stats = Table('dag_stats', Base.metadata)
    dag_stats.drop(settings.engine, checkfirst=True)
    Base.metadata.drop_all(connection)
    
    Base.metadata.remove(dag_stats)
    Base.metadata.remove(users)
    Base.metadata.remove(user)
    Base.metadata.remove(chart)
    
    from alembic.migration import MigrationContext
    migration_ctx = MigrationContext.configure(connection)
    version = migration_ctx._version
    if version.exists(connection):
        version.drop(connection)
def drop_flask_models(connection):