NMS-15852: Reduce minimal pool size

[fooker]

As we now spawn two pools (by self monitoring DB stats collection), we exceed the psql default max connection count (of 100) easily. As a first mitigation, we reduce the number of connections to create on startup and assume that a single collector will never increase the pool size.

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External References

• Jira (Issue Tracker): https://opennms.atlassian.net/browse/NMS-15852