Redshift maintenance macro improvements

[clrcrl]

1. Check that the relation exists before trying to vacuum this. This can be a source of failures for long running vacuums
2. Create logic for "respect my project's get_vacuumable_tables macro, otherwise, use the default implementation

{% macro redshift_maintenance() %}
    {% if context[ref.config.project_name].get_vacuumable_tables %}
        {% set vacuumable_tables=context[ref.config.project_name].get_vacuumable_tables() %}
    {% else %}
        {% set vacuumable_tables=redshift.get_vacuumable_tables() %}
    {% endif %}

    {% for table in vacuumable_tables %}
       ...
    {% endfor %}

{% endmacro %}

[clrcrl]

Closed by #25