expose `reconnect` option for mysql connector

Allow reconnect to be configured for database.yml

reconnect is an option for the mysql2 client, that we actually have included in our source database.yml for mysql here: https://gitlab.com/gitlab-org/gitlab-ce/blob/master/config/database.yml.mysql

this may workaround an issue a customer is seeing for sidekiq freezing when it loses the database connection: https://gitlab.zendesk.com/agent/tickets/45666

we need to investigate whether we can use reconnect for GitLab, as it reverts any previous transaction that the connection had running: http://dev.mysql.com/doc/refman/5.7/en/auto-reconnect.html