refactor: Split config/halfstack/global contexts from root context

[achimnol]

This PR splits RootContext into three categories of contextual objects:

• Configurations (those from etcd and local TOML files, runtime identifier like pidx)
• Halfstack connection pools (postgres and redis)
• Global stateful objects (event bus, background task manager, storage proxy connector, etc.)

Relationship:

• replaces fix: Miscalc and inefficient db access patterns of session concurrency limits #3064
• resolves Refactor RootContext to explicitly distinguish context info groups #3156 (BA-37)

Checklist: (if applicable)

• Milestone metadata specifying the target backport version
• Mention to the original issue

[achimnol]

• fix: Rewrite concurrency tracking mechanism to use Redis sets #3155
• refactor: Split config/halfstack/global contexts from root context #3154 👈 (View in Graphite)
• main

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