Feature/improves multiple databases support

[abxsantos]

Improvement related to issue #558.

• Added a Dockerfile.dev to ensure a standardized development environment setup
• Improved multiple database support (read/write replicas and multiple databases)
• Added unit test scenarios for multiple database support to the scheduler
• Updated documentation with the has_replica key allowing better multidb support

[Koed00]

Thanks for the work on this!

[abxsantos]

It was a pleasure!

[mohmyo]

I have just updated to 1.3.6 and my logs started to show a persistent error for every task or schedule run:
19:00:42 [Q] Select_for_update cannot be used outside of a transaction.

Reverted back to 1.3.5 and it is working like normal again.

I'm using ORM with different DB to read and write, defined my DB router for read and write, no replica or anything else, here is my setup:

Q_CLUSTER = {
    'name': 'background_tasks',
    'workers': 4,
    'timeout': 90,
    'daemonize_workers': False,
    'retry': 3600,
    'compress': True,
    'recycle': 100,
    'max_rss': 60000,
    'save_limit': 0,
    'queue_limit': 50,
    'bulk': 10,
    'orm': 'tasks',
    'error_reporter': {
        'sentry': {
            'dsn': config('SENTRY_DSN')
        }
    }
}

routers.py

class Django_Q_Router:
    """
    A router to control all database operations on models in the
    django_q application.
    """

    def db_for_read(self, model, **hints):
        """
        Attempts to read django_q models go to tasks.
        """
        if model._meta.app_label == 'django_q':
            return 'tasks'
        return None

    def db_for_write(self, model, **hints):
        """
        Attempts to write django_q models go to tasks.
        """
        if model._meta.app_label == 'django_q':
            return 'tasks'
        return None

    def allow_migrate(self, db, app_label, model_name=None, **hints):
        """
        Make sure django_q app only appear in the
        'tasks' database, other apps on 'default'.
        """
        if app_label == 'django_q':
            return db == 'tasks'
        else:
            return db == 'default'

I believe this fixes the problem and ensure the correct way to do this considering all different use cases.
here: https://github.com/edthrn/django-q/pull/1/files

@Koed00