CBG-4116 skip successful auth on silent handlers

[torcolvin]

• Report an authentication failed event if no credentials are specified on the admin port
• /_ping endpoint uses publicPrivs so it doesn't log auth event anyway.

Tentatively removing

if !base.UnitTestUrlIsWalrus() {
		t.Skip("This test can panic with gocb logging CBG-4076")
}

but I should possibly pull that out into a separate commit. Without removing this, the test doesn't work! I'm not sure that we have a way to use rosmar with admin auth. I'm trying to reproduce CBG-4076 again.

I have a separate commit that makes all loggers work on an atomic.

Pre-review checklist

• Removed debug logging (fmt.Print, log.Print, ...)
• Logging sensitive data? Make sure it's tagged (e.g. base.UD(docID), base.MD(dbName))
• Updated relevant information in the API specifications (such as endpoint descriptions, schemas, ...) in docs/api

Integration Tests

• GSI=true,xattrs=true https://jenkins.sgwdev.com/job/SyncGateway-Integration/2619/