Implement incremental models with "merge" on Snowflake

[drewbanin]

Feature

Feature description

dbt's incremental models should use a merge statement to incrementally load data into a table.

If no unique_key is provided, dbt should use a simple insert statement, as Snowflake doesn't like merging on a constant false predicate. More info here

This is a backfilled issue for some currently-open PRs that implement the functionality described in this issue:

• Adding incremental logic to Snowflake plugins #1307
• Snowflake create or replace #1409

See also: #1101

[drewbanin]

Fixed in #1409