logging: Fix skip_hosts with wildcards

[francislavoie]

Fix #4859

See #4859 (comment) for an explanation of the bug.

The fix was simple, really, we just need to reorder the loops in server.go to return true early if a logger is explicitly configured for a hostname.

While testing it though, I noticed that there's a bug in the Caddyfile adapter when using the log directive with no options (i.e. log to the default logger); it wasn't adding those hosts to the logger_names config, so the wildcard skip_hosts would still cause that to get skipped when it was explicitly configured to log.

Basically, I removed some code because it was trying to be "too smart" with omitting some config in the output JSON for "cleanliness" but that just causes edgecase bugs.

Also a side benefit is that logger checks should be slightly faster now (like... nanoseconds at best) because we check a map in O(1) before looping over skip_hosts which is O(n).

Also set up a const in the caddy package for the default logger name, because we use that in lots of places.

[mholt]

Huh, thanks! I like fixes that delete code. Thanks for correcting my mistake. 😁 👏