[BUG] Running Kiln in a HA configuration causes message duplication

[DanHatesNumbers]

Describe the bug
When running the report-parser and slack-connector components with more than one instance, sending a ToolReport to Kiln will result in duplicate messages being sent to Slack. When tested with 2 report-parser instances and 2 slack-connector instances, messages were replicated an additional 3 times.

To Reproduce

1. Follow instructions in Integration Testing guide to prepare a local Kiln stack, but instead of running the documented command to start the report parser and slack connector, run docker-compose up --scale report-parser=2 report-parser and docker-compose up --scale slack-connector=2 slack-connector.
2. docker run -it --net=host -v "${PWD}:/code" -e SCAN_ENV="Local" -e APP_NAME="RailsGoat" -e DATA_COLLECTOR_URL="http://localhost:8081" kiln/bundler-audit:master-latest
3. Observe 4 messages posted for each vulnerability found

Expected behavior
Each vulnerability in Bundler-audit output is posted to Slack once

Screenshots
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Environment details

• Component: Report parser and Slack connector
• Commit: 6c2acb4

Additional context
Possibly because the kafka crate we're using doesn't seem to handle multiple consumers in a group properly, because calling consumer.commit_consumed() "Persists the so-far "marked as consumed" messages (on behalf of this consumer's group for the underlying topic - if any.)"
Which I think means the whole topic, not just the partitions this consumer is subscribed to

There's an alternative Kafka crate which wraps the librdkafka C library, which as a bonus also supports the new async await syntax (in master fede1024/rust-rdkafka#187, which the maintainers said should be released in a few days)
Based on their docs, that crate seems to handle committing consumed messages within a topic partition correctly