Snapshots run many introspection queries

[drewbanin]

Describe the bug

Snapshots run many introspective queries. These queries:

• Find the columns in the destination table
• Find the columns in the staging table
• Add columns as needed
• Expand column types

Some of these queries are required, but others are a little bit duplicative and a function of the implementations of the adapter methods that are called in the snapshot task.

Steps To Reproduce

Run dbt snapshot and inspect the logs

Expected behavior

Let's deeply investigate the queries initiated by a dbt snapshot command and understand which are necessary and which are a function of the abstractions in the existing adapter function implementations.

See the comment here #2390 (review)

The output of dbt --version:

= 0.17.0, but this hasn't changed in a while

[github-actions]

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