Shard MySQL for connect four context

[marein]

This pull requests applies schema-based sharding to the connect four context. It plays nicely with ProxySQL (which will be added for the prod example later). The env variable APP_CONNECT_FOUR_DOCTRINE_DBAL_URL needs to point to ProxySQL instead of MySQL, APP_CONNECT_FOUR_DOCTRINE_DBAL_SHARDS should define the logical shards and APP_CONNECT_FOUR_DOCTRINE_DBAL_DATABASE needs to point to the first shard. The following ProxySQL service definition routes 3 logical shards to 2 physical shards once #167 is merged.

Show code

proxysql:
    image: marein/php-gaming-website:proxysql
    environment:
        PROXYSQL_CONFIG: |
            admin_variables: {
                restapi_enabled="true"
            }
            mysql_variables: {
                server_version="8.0"
                monitor_username="root"
                monitor_password="password"
                auto_increment_delay_multiplex=0
            }
            mysql_servers: (
                {hostgroup=1,address="mysql-1",port=3306,max_connections=100},
                {hostgroup=2,address="mysql-2",port=3306,max_connections=100}
            )
            mysql_users: (
                {username="root",password="password",default_hostgroup=1}
            )
            mysql_query_rules: (
                {rule_id=1,schemaname="connect-four-shard1",destination_hostgroup=1,apply=1},
                {rule_id=2,schemaname="connect-four-shard2",destination_hostgroup=2,apply=1},
                {rule_id=3,schemaname="connect-four-shard3",destination_hostgroup=1,apply=1},
                {rule_id=4,match_pattern="^CREATE DATABASE(?: IF NOT EXISTS)? `connect-four-shard1`$",replace_pattern="CREATE DATABASE IF NOT EXISTS `connect-four-shard1`",destination_hostgroup=1,apply=1},
                {rule_id=5,match_pattern="^CREATE DATABASE(?: IF NOT EXISTS)? `connect-four-shard2`$",replace_pattern="CREATE DATABASE IF NOT EXISTS `connect-four-shard2`",destination_hostgroup=2,apply=1},
                {rule_id=6,match_pattern="^CREATE DATABASE(?: IF NOT EXISTS)? `connect-four-shard3`$",replace_pattern="CREATE DATABASE IF NOT EXISTS `connect-four-shard3`",destination_hostgroup=1,apply=1}
            )
    depends_on:
        - mysql-1
        - mysql-2

Tested with 3 physical MySQL shards and ProxySQL deployed as a sidecar of the app. The test was running for 10 minutes with a constant throughput of 25k write requests / s. The load was well distributed.

This pull request doesn't take resharding into consideration.