Add topic+tag index to messages

[peterbroadhurst]

The intention of the topics field, is that you can create a stream of data that is grouped on a particular ordered business context, and the tag sub-classifies messages within that.

This means that querying by topic and tag is a primary use case, for which index(es) should exist.

This PR proposes a single combined index, on topic then tag, as I believe that's the most likely combination of filter across use cases.

[codecov-commenter]

Codecov Report

Merging #290 (84a3a97) into main (70d1022) will not change coverage.
The diff coverage is n/a.

❗ Current head 84a3a97 differs from pull request most recent head 545eae5. Consider uploading reports for the commit 545eae5 to get more accurate results

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