Use Storage-Attached Indexing (SAI) as alternative eventsByTag implementation

[patriknw]

The new Storage-Attached Indexing (SAI) looks like a strong candidate for eventsByTagQuery, (eventually) replacing our custom tag_views table.

Resources:

• https://sdtimes.com/data/apache-cassandra-indexing-without-having-to-say-im-sorry/
• https://docs.datastax.com/en/storage-attached-index/6.8/sai/saiTOC.html
• https://docs.datastax.com/en/astra/docs/using-storage-attached-indexing-sai.html
• https://www.datastax.com/dev/cassandra-indexing

Note that SAI is not available in OSS Apache Cassandra yet, but according to CEP-7 the intention is to include it in Apache Cassandra.
It is included in DataStax Astra and DataStax Enterprise.

[patriknw]

@chbatey Do you see any obvious reason why SAI wouldn't be a good fit for this?

[patriknw]

I guess we would define two indexes. One on the tags and another on the offset (timestamp). Looks like query with two AND clauses could work: https://docs.datastax.com/en/storage-attached-index/6.8/sai/saiFaqs.html#saiFaqs__saiAndQueriesFaq