Ensure that snapshot valid_from and valid_to timestamps are non-overlapping

[drewbanin]

From Josh P and Claus H in Slack:

we were using the dbt_valid_from and dbt_valid_to ranges to determine which row represents the correct values at a point in time, but now it means 2 rows would be selected for finding a point-in-time between the 3 seconds overlap of the example i posted

dbt should instead ensure that snapshot change records for the check strategy are non-overlapping.

Sample code:

{% macro snapshot_check_fixed_strategy(node, snapshotted_rel, current_rel, config, target_exists) %}
    {% set check_cols_config = config['check_cols'] %}
    {% set primary_key = config['unique_key'] %}

    {# -------------- NEW ------------------ #}
    {% set select_current_time -%}
        select {{ snapshot_get_time() }} as snapshot_start
    {%- endset %}

    {% set now = run_query(select_current_time)[0].snapshot_start %}
    {% set updated_at = "'" ~ now ~ "'::timestamp" %}
    {# -------------- NEW ------------------ #}

    {% if check_cols_config == 'all' %}
        {% set check_cols = get_columns_in_query(node['injected_sql']) %}
    {% elif check_cols_config is iterable and (check_cols_config | length) > 0 %}
        {% set check_cols = check_cols_config %}
    {% else %}
        {% do exceptions.raise_compiler_error("Invalid value for 'check_cols': " ~ check_cols_config) %}
    {% endif %}

    {% set row_changed_expr -%}
        (
        {% for col in check_cols %}
            {{ snapshotted_rel }}.{{ col }} != {{ current_rel }}.{{ col }}
            or
            ({{ snapshotted_rel }}.{{ col }} is null) != ({{ current_rel }}.{{ col }} is null)
            {%- if not loop.last %} or {% endif %}

        {% endfor %}
        )
    {%- endset %}

    {% set scd_id_expr = snapshot_hash_arguments([primary_key, updated_at]) %}

    {% do return({
        "unique_key": primary_key,
        "updated_at": updated_at,
        "row_changed": row_changed_expr,
        "scd_id": scd_id_expr
    }) %}

{% endmacro %}

The sample code shown above will need to be updated to:

1. use database-specific quotes (not ' everywhere)
2. use a database-specific timestamp type