Introduce SessionMap to track per-lb sessions

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.

Follow-up of !166566 (comment 2125393042)

~360 LoC are from new code. The other are more or less drop-in replacements.

Issue: gitlab-com/gl-infra/scalability#3834 (closed)

Required Approvals

Reviewers

• backend @reprazent
• database @stomlinson

Maintainers

• backend @nbelokolodov
• database @ahegyi
• clickhouse @ahegyi
• verify @hfyngvason
• groupauthentication @ifarkas

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