**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
"""Upgrade the database."""
    
    from alembic import command
    log.info("Creating tables")
    config = _get_alembic_config()
    config.set_main_option('sqlalchemy.url', settings.SQL_ALCHEMY_CONN.replace('%', '%%'))
    
    errs = auto_migrations_available()
    if errs:
        for err in errs:
            log.error("Automatic migration is not available\n%s", err)
        return
    with create_global_lock(session=session, pg_lock_id=2, lock_name="upgrade"):
        command.upgrade(config, 'heads')
        add_default_pool_if_not_exists()
@provide_session
def resetdb(session=None):
    """Clear out the database"""
    log.info("Dropping tables that exist")
    connection = settings.engine.connect()
    with create_global_lock(session=session, pg_lock_id=3, lock_name="reset"):
        drop_airflow_models(connection)
        drop_flask_models(connection)
        initdb()
def drop_airflow_models(connection):