Deduplicate webhook headers on subsequent attempts

This also updates the database with the final payload headers, including
the signature and nonce, which are generated afresh for each attempt.

webhooks/legacy: increase specificity of logging

workers/legacy: correct Println with fmt directive

Make worker logs less loud

webhooks/legacy: address Postgres-specific issues

webhooks/legacy_test.go: style

webhooks/legacy: update only affected delivery

Forgot to add this where clause, whoops!

Add legacy webhooks worker implementation