**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
"""Add new Connection."""
    if not session.query(Connection).filter(Connection.conn_id == conn.conn_id).first():
        session.add(conn)
        session.commit()
@provide_session
def add_default_pool_if_not_exists(session=None):
    """Add default pool if it does not exist."""
    if not Pool.get_pool(Pool.DEFAULT_POOL_NAME, session=session):
        default_pool = Pool(
            pool=Pool.DEFAULT_POOL_NAME,
            slots=conf.getint(section='core', key='default_pool_task_slot_count'),
            description="Default pool",
        )
        session.add(default_pool)
        session.commit()
@provide_session
def create_default_connections(session=None):
    """Create default Airflow connections."""
    merge_conn(
        Connection(
            conn_id="airflow_db",
            conn_type="mysql",
            host="mysql",
            login="root",
            password="",
            schema="airflow",
        ),