patch redshift table size estimation

[vogt4nick]

resolves #2702

Description

Root cause of #2702 seems to be size in MB is passed to format_bytes which expects amount in bytes. This PR estimates Redshift table size in bytes.

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.

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[beckjake]

This looks good to me, thank you for contributing to dbt!