[ML] Investigate / fix degraded anomaly detection performance on specific QA test sets

[jan-elastic]

The CStringStore removal has caused degraded performance on a few QA test sets (next to improved / neutral performance on many others, so this doesn't warrant reverting). Investigate what causes this and, if possible, fix it.

The affected test sets are:

• nginx-remote-ip-request-rate
• nginx_remote_ip_url_count
• iowa_store_liquor_sales_pop

See also: this QA issue.

Note: no slowdown has been observed locally on MacOS, so this probably needs investigation on Linux or Windows.

[ricor07]

Hello, I'd like to join the project and work on this.