Allow to wait some time before timeouting initial connection to the database

I'm running this in local docker environment and I'm hitting an error which says that database refuses connection:
```
web_1 | worker.1 | Error: SQLSTATE[HY000] [2002] Connection refused
web_1 | worker.1 | #0 /var/www/project/vendor/humanmade/cavalcade-runner/lib/Runner.php(200): PDO->__construct('mysql:host=mysq...', 'web', 'password', Array)
web_1 | worker.1 | #1 /var/www/project/vendor/humanmade/cavalcade-runner/lib/Runner.php(89): HM\Cavalcade\Runner\Runner->connect_to_db()
web_1 | worker.1 | #2 /var/www/project/vendor/humanmade/cavalcade-runner/bin/cavalcade(26): HM\Cavalcade\Runner\Runner->bootstrap('/var/www/projec...')
web_1 | worker.1 | #3 {main}
```
This happens because my mysql is not ready yet. Could we add a default timeout which could be adjusted with an flag like `--timeout=10`?

Start with lib/Runner.php, especially connect_to_db(), and the bin/cavalcade entry point shown in the stack trace. Trace how the initial PDO connection is made and determine how the requested --timeout=10 option would be passed through. Done means startup can wait for a delayed MySQL connection using an adjustable default timeout.

Written by the indexing model from the issue text.