change(log): Silence verbose failed connection logs

[teor2345]

Motivation

1. Having lots of failed connection logs makes Zebra less usable, but when there are only a few connections users need to know why
2. It's also causing GitHub to be very slow showing logs in tests

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Solution

• In production, log failed connections at info level at startup, and when there are less than 5 connections
• In tests, always log failed connections at debug level (except maybe for startup)

Review

This is a low priority logging fix.

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• Are there enough tests to make sure it works? Do the tests cover the PR motivation?
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[arya2]

Looks good!