Use SessionMap for tracking per-db primary/replica usage

Sylvester Chinrequested to merge
sc1-release-session-map into master

What does this MR do and why?

This MR releases the change introduced earlier by removing the feature flag. Primary/replica usage of each connection is now tracked separately.

For context, the ff rollout effects are summarised in gitlab-com/gl-infra/production#18673 (comment 2170397121) and gitlab-com/gl-infra/scalability#3834 (comment 2177079060)

Original MR: !168642 (merged)

See gitlab-com/gl-infra/scalability#3834

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Edited by Sylvester Chin

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