Introduce SessionMap to track per-lb sessions - 2nd try

What does this MR do and why?

We introduce a Gitlab::Database::LoadBalancing::SessionMap which holds the hash of lb-name to Gitlab::Database::LoadBalancing::Session mapping.

NOTE: This MR is a 2nd attempt of the reverted !166986 (merged). The spec and missing feature-flag path is added as 662b280e

Issue: gitlab-com/gl-infra/scalability#3834

Required approvers

reviewers

• backend @reprazent
• database @stomlinson

maintainers

• backend @vshushlin
• database @a_akgun
• clickhouse @vshushlin
• Verify @tianwenchen
• groupauthentication @dblessing

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