Dont drop tables in incremental append runs

[stephen-up]

Hi, when dbt runs for a incremental+append it always drops the temporary tables that are used for full refreshes. Even if its not going to use them.

In my system, I have scheduled incremental runs of dbt that run frequently updating incremental+append tables. Whenever I do a full refresh (concurrently), the scheduled run drops the tables of the full refresh causing it to crash.

This PR stops that, by making the incremental+append runs not drop tables.

I see there have been a few other issues similar to this, but this seems like an easy minimal way to fix the issue.

• Randomize temp table __dbt_new_data name to allow concurrent dbt-clickhouse jobs #150
• Make temporary table names unique in incremental materializtion #353

Thanks

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[stephen-up]

I signed the CLA btw. Dont know how long it takes to update.