[wolf4ood]

What this PR changes

More tuning guidance from the 1.0 load tests with several controlplane replicas:

• Database connections: keep max-idle close to max-total. With the default of 4 the pool keeps closing and
  reopening connections under bursty load, which cost PostgreSQL about a third of its CPU in the load test.
• Credential service (new): with DCP every incoming protocol message triggers a presentation query to the
  counter-party's credential service, so an IdentityHub serves the protocol traffic of all the participants it hosts.
  With its default pool of 8 connections it capped the whole platform.
• Finding the limit: how a saturated credential service shows up in thread dumps.
• Replicas need CPU of their own to add throughput.
• Tracing has a measurable cost under load.

Why it does that

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Further notes

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signature changes, package declarations, bugs that were encountered and were fixed inline, etc.

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Linked Issue(s)

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