Capture hard deleted records in snapshots

[jtcohen6]

Resolved #143

Alternative: We stop overriding the default merge macros. Databricks' implementation is strictly better because it lets us use update * + insert *, but the default ones should still work, and it would avoid situations like this one where we miss an upstream improvement.

Checklist

• I have signed the CLA
• 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
• I have updated the CHANGELOG.md and added information about my change to the "dbt next" section.

[jtcohen6]

I really went back and forth on this: Should we just use the default versions of get_merge_sql and snapshot_merge_sql, since they already work on Delta? We need to override the incremental and snapshot materializations for other reasons, though, so it's not like we could get away with the full default implementation. I've decided to keep the change minimal for now.