Snapshot time data type

[gshank]

resolves #242

Problem

The values of the timestamp columns in a snapshot are different than the source table, leading to unexpected results.

Solution

Add comparison of 'snapshot_get_time' and snapshot config 'updated_at' columns and issue a warning.

Note: test case is in dbt-labs/dbt-core#10352

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