Allow use of multiple column unique keys in snapshots

[gshank]

resolves #181

Problem

Implement use of a list for a unique_key in snapshots

Solution

Check whether the unique_key is a list and modify the SQL.

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development, and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc.) or this PR has already received feedback and approval from Product or DX